This wizard needs complete immunity to the aboleth's ability
Based on what you've described, something that gives a simple bonus to or advantage on saving throws isn't going to be sufficient. Every time the wizard visits the aboleth, the aboleth will get 3 chances to enslave him, so he will want to leave nothing to chance. This means that the wizard must accept nothing less than complete protection against the aboleth's enslavement.
I don't see any items that will do the job
There are a few magic items that can end the charmed condition under certain circumstances, but these will not be able to reliably end the aboleth's enslavement, because the aboleth will not allow the wizard to use them while enslaved. It might work once or twice while the aboleth is unaware of the item, but an aboleth is smart and will catch on quickly.
There is the Staff of the Forgotten One, which confers immunity to charm among other things, but the staff is an artifact, it is canonically in the possession of a specific legendary undead, and it has some... let's say not insignificant downsides. This probably isn't something you want to potentially let fall into the hands of a low-level party, unless you want to cause a TPK. There is also a Greater Silver Sword, but this is a legendary +3 weapon with a special ability involving the astral plane -- again, probably not appropriate for levels 1-6.
A few wizard spells can help here
As other answers have pointed out, Mind Blank makes one creature immune to the charmed condition (not to mention all the aboleth's telepathic abilities) for 24 hours, no concentration required. This is clearly the superior solution, but it is also an 8th level spell, which means your level 1-6 party isn't going to have access to it unless you give it to them somehow. There is also the 6th level spell Tasha's Otherworldly Guise, which can grant immunity to the charmed condition (among other benefits) for 1 minute, concentration required. This would be sufficient for a short conversation, but little else.
However, there is one other suitable spell in this specific case, because the aboleth happens to be an abberation: Protection from Evil and Good, a 1st level spell that, among other things, makes the target immune to being charmed by abberations. This spell lasts 10 minutes and requires concentration, and it is a wizard spell as well as a cleric and paladin spell, so not only can your wizard NPC have the spell, the party healer can probably prepare it given advance warning of what is to come. The 10-minute time limit is long enough to have a significant conversation, but also short enough to impose some significant time pressure, and the requirement for concentration gives the aboleth the possibility of breaking the caster's concentration in order to get a chance at enslaving them, so if the players use this solution, you as the DM have plenty of opportunity to create tension during the encounter.
The downside of all of these spells, however, is that they can only protect 1 creature each, which is fine for your wizard NPC but not so great for warding the entire party.
The wizard's best defense is knowledge
Assuming the wizard's research has effectively given him complete knowledge of the aboleth's abilities, there are 2 very straightforward ways to defend against the aboleth's enslavement:
Enslave (3/Day). The aboleth targets one creature it can see within 30 feet of it.
In order to use this ability, the target must be visible to the aboleth, and it must be within 30 feet. So, as long as the wizard remains invisible or outside the range of this ability, the aboleth will be unable to even attempt to enslave him. As for how to accomplish this in practice, aboleths are quite mobile in water but only have a speed of 10 feet on land, so the wizard should not come within 40 feet of any water (10 ft movement + 30 ft range). If the aboleth has any sense, there will be no such safe point anywhere within its lair, so most likely this will mean communicating from a safe distance outside the lair, perhaps via the Message spell, which has a range of 120 feet, or perhaps a familiar carrying notes back and forth. (After any communication session, the familiar should of course be considered compromised and the wizard should dismiss and re-summon them.)
Alternatively, if especially brave, the wizard can cast Invisibility on himself and enter the aboleth's lair. The aboleth has a +10 perception modifier, so the wizard will probably not go undetected, but since the aboleth does not have truesight or any other ability to see invisible creatures, it will be unable to target the invisible wizard with its enslavement. Of course, the wizard must still remain at least 20 feet from any pools of water, or else the aboleth will be able to either attack him (10 ft land speed + 10 ft reach) or use its grasping tide lair action to pull him into the water (and then attack him). It is imperative to avoid any possibility of attack, since this has a chance to break the wizard's concentration on Invisibility.
Of course, any spell or ability that allows long-distance communication would be ideal. This includes spells like Sending and Dream. Another option for long-distance communication with an aboleth specifically would be to capture and imprison one of the aboleth's thralls, because the aboleth has a bidirectional telepathic link with its thralls over unlimited distance. Note that the thrall should be subdued without dealing damage to them, since they get a saving throw each time they take damage. The Sleep spell will likely be useful for this purpose. If the aboleth is amenable to working with the wizard, it can use the captured thrall as a messenger. As the DM, it is of course your job to come up with a situation where the aboleth would be willing to entertain this idea.
Knowledge is shareable with the party
You stated that ideally the wizard's means of defending against the aboleth's enslavement should be usable by the PCs. Fortunately, the knowledge of how to stay out of range of the effect is something that the wizard can share with the PCs, or something the PCs could discover by reading the wizard's notes. Usually PCs facing an aboleth for the first time don't know the precise limitations of its abilities, so giving this information to the players can give them a significant tactical advantage. And unlike many spells and items, this information benefits all the PCs. Of course, the wizard (or his notes) notes should also clearly indicate that the aboleth's enslavement is a charm effect, because the (non-wizard) PCs will likely have good ways of negating such effects.