By allowing your players to get more feats at level 1, you allowed access to a prestige class earlier than you should have, as normally he would only be able to get the first level of Forsaker at level 4 and that is only if he is human and at this point he would only have Ability bonus +1, fast healing 1 (10 max), forsake magic and SR 11, which are not that big deal for a level 4.
If he is from any other race that doesn't grant a free feat at level 1, he would need 3 more levels, qualifying at lvl 6 and getting the first level at level 7, getting the same features the human would get, but later.
These class features were intended to make a Forsaker as powerful as someone that gets the right amount of magical items for each level, but its main flaw is that the Forsaker doesn't have to deal with items not being what he needs.
So I urge you to rethink about allowing him to have this PrC at this point, my other points assume you're sticking with your first decision of allowing him to have the PrC at this point.
I don't recommend giving more magic loot to other players to compensate them for the Forsaker being OP as this could create the idea that the "game master is out to get me" or that you can't handle the Forsaker as a GM and what you actually want is for him to realize that playing a magic item addict that doesn't get the benefits of magic items is annoying even if the player believes otherwise.
Game world economy doesn't work in his favor. Magic items are not always available (specially in the wilds or outside of places where they are usually accessible) and if i were the GM i wouldn't let him store magic items to break when he needs it as it's against the whole philosophy of this prestige class.
He has to go to the 'magic item mall' every day while not adventuring and sometimes while adventuring (which in itself is quite weird/contradictory a person that hates magic shopping for magic items), spending his gold there, this may slow down the game because he has to go to town to buy his 'juice'. He also can refuse to use magic mounts, fly by magic means or be teleported and even go to magical places which can slow down games even more (be sure to punish him if he accepts to use magic indirectly).
Making things a bit difficult
Now that i got my 4 main points we can go through weaknesses that can be exploited without you changing the prestige class, but be reminded that you can change the class, i.e. change his DR to only work versus magical effects, also, like other people mentioned before me, you can always create challenges that are outside his powers, but I presume this is not the problem, you actually want HIM to be challenged in combat.
If by any chance there isn't enough magic items for him to destroy (remember, every point of DR requires 100 gp, so at level 3 he needs to spend 300gp every day), he will have no DR... suddenly he is way less tankier than usual, also, if that goes on for a bunch of days, he may be tempted to destroy the magic items in possession of his companions, after all he is a against anything magic and you can play this as plot material if you are comfortable playing the players against themselves. Also, if he doesn't destroy magic items right after he acquires them, you can have his enemies steal his 'fuel' to make him vulnerable.
If you want to challenge him during combat and using spells you can use damage spells that have the entry Spell Resistance: No, like: Melf's Acid Arrow (which has the extra benefit of being a ranged touch attack, so it makes his armor useless) or the various Orb of spells from Complete Arcane, or use Effect spells as most of them can't be resisted with Spell Resistance, like: Grease, Web, Fog Cloud and Darkness.
He is not immune to traps. He can still fail a check and fall on a moat or a tar pit or anything else and if he is alone, his anti-magic resources do nothing to help him, so if he is a gung-ho type of character that doesn't prepare himself (like buying ropes, rations, etc.) he can get screwed easily because magic is also used to cover for these things (Ex: he probably should refuse to eat food generated by magic, so he would be forced to carry his food or eat whatever he finds).
You can also make him busy in combat through Summon spells, as they obviously don't target him and create more enemies to deal with, specially if you have a summoner with Augmented Summoning feat (boosts Str and Con of summoned creatures by +4).
Also, feel free to let the enemies use buff spells (True Strike, Invisibility, Magic Weapon, etc.) to allow them to bypass his defenses or make his life difficult (Miss chance, enemies hitting his high AC on a roll of a 7 or simply ignoring his DR due to having +1 weapons is annoying to someone used to roflstomping enemies without consequence). And he can't give any excuse for them using magic against him as he actually needs to have magic-using people as enemies.
Use the combat maneuver feats (Improved Trip, Grapple, Feint, Disarm, Sunder, etc.) to waste his actions. Some enemies don't need to deal damage to wreak havoc. Disarming or Sundering is pretty good if he is not using his fist as a weapon, because his bonus derived from DR only counts for hitting/bypassing DR (so they don't increase his damage) and his punches will provoke Attacks of Opportunity and roll low dice for damage unless he goes the route of unarmed attacks which isn't better than being a Monk.
Damage reduction does not negate touch attacks, energy damage dealt along with an attack, or energy drains. Nor does it affect poisons or diseases delivered by inhalation, ingestion, or contact.. (This is actually similar to traps as they are mundane things that can hurt him and even kill him).
His extra attribute points focused on Constitution make him a damage sponge, but his constitution can be lowered through weapons with keyword Wounding ( I only recommend to do that if he gets famous/infamous and more clever enemies).
Forsaker was not designed to be achieved easily or in all kinds of campaigns, i.e. low-magic games can't sustain a Forsaker because of magic scarcity. And it can be argued that he is not forsaking anything as he is actually using magic items as fuel and his powers are very magic-like.
In any game world where magic and magic items are relatively abundant, if this guy becomes a bandit or if there is a criminal faction that is related to magic or magic items, they would surely use their power to eliminate him before he gets powerful enough to become a threat to them.