It depends on the stage you set up
Given your high powers, you can play like a screenwriter or level designer, where every part is carefully designed to give advantages to your party, while making it look like it is actually not the case. Because, all in all, it's a lot about setting up the stage of the scene. And the quality of that stage will determine how much support spells you can give without anyone noticing. Very little without, almost anything with.
Setting a stage like this will depend on your grandson's mission, but it is possible to find common elements that help you in keeping control of the events and that you can adapt to the quest. Here are some examples :) :
Note of friendliness : if the grandson and his party is controlled by other players, proceed with caution with dominate person spells, killing people or trapping them for indefinite periods of time. It is generally not considered as fun to lose control of your character for long.
Splitting the group to gain control over your grandson
You cannot help the group as a whole, but nothing says you cannot hinder the group to your advantage! Force them to separate, through an enemy's looking trap, by the appeal of gold, internal group conflict or anything you like. If you're really evil, just kill the other party members, they're just bothering you and your plan anyway :). Otherwise trap them and prevent them to join your grandson. That's important.
This will result in only having the grandson to care about. You should be able to easily take him by surprise and cast "dominate person" on him so that he doesn't realize the tricks which will happen afterwards. Whether or not the effect of the spell is felt, it most probably won't be felt as help (being dominated isn't very nice), and you can immediately order him to not notice you, your spells, and anything you do (except obeying your orders, of course).
After that, boost him for the enemy encounters he might find with your favorite support spells, items and such, and send him to finish the quest.
Finally, clean the scene. Remove some of the dead bodies and put his comrade bodies if you decided to kill them, and ensure that your grandson doesn't have lasting spells or items he didn't have before. Ideally, you want to have him hurt to give more credibility. The idea is to make the fights believable to the eyes of the other members (if alive) and the grandson. Finally, flash out his memories with the "modify memory" spell, replacing them with ones corresponding with the scene that you made. Then release everything.
If you decide to kill the other party members, it will give the great benefit of making it feel it was no easy quest, add more flavour the scene, and the grandson will think more about his deceased companions than suspecting someone helped him. Indeed, how can someone has helped him when this person let his companions die?
Make it looks like a curse
Give an item containing a fake "curse" to your grandson, through a bargain price at the market for instance. To make it sure it works, play the shopkeeper with the spells "alter self", "suggestion", and "vocal alteration" to ensure he takes the item without knowing it comes from you. The item itself don't need to contain a lot of magic or power, it could really be a basic helmet with just shinier horns than usual.
When carried, cast dominate person on him and prevent him from putting down the item, in order to fake the curse and keep your magician's wand to distract what really happens. When the danger arises, force him to act berserk like he got controlled by a demon. And give him the boosts suited for that inexistent demon.
After the mission, end the "curse" by using a summon spell to make it look like the demon was freed. Or dare to play the role yourself with a polymorph spell! In any case don't forget your evil speechies and dark sparkles. You don't want them to think this demon or whatever it is helped them.
"Help" the enemies
That's another counter-intuitive one, and you need to know what you are doing. You cannot put a legendary sword lying on the floor like this, but you can always give magic items to enemies, so that once dead, give advantages to your grandson's party.
This can be especially believable if said enemies are known scavengers or raiders, but in order for this to work you also need to take care said item is not compatible to the enemy, otherwise you will only make the fights harder :p! For instance, "give" (as in "let them know it's there") an heavy, human-sized full-platemail to goblins who cannot possibly wear it or lawful-good restricted items to chaotic-evil demons.
Then just lure the group towards where the item is be it with illusions, suggestions or bluff. Hopefully it will be inside a relatively unguarded stash. In the bad event it is carried by an enemy, charm them into putting the item down in a better spot. As long as the action has no impact on the enemy's state/position when the challenge start, it's not weakening them!
Alternatively, as stated in the comments, you can lay your equipment near an unaware or resting soldier, and help the team at this easy picking! It would look like they were the one to put them down, and not you.
To finish this, here are some spells that can be helpful :
- Dominate person : If you're willing to control your own grandson, if that is.
- Greater invisibility : An almost must-have, otherwise you won't be able to cast anything. Beware of the noise though! You might want the silent spell's feat with it.
- Screen : An alternative to greater invisibility, allowing you to stay quiet in the zone, too.
- Wish : avoid fatal strikes, or make a blow have enough impact to kill the bad guy. It can also helps in reducing spell saves on your grandson in order for other support spells. A life saver. And quite believable in the direst of situations.
- Image (programmed, minor, major and permanent) : Guide them through obstacles with illusions. Do you know where the trap is? Then bar the door to it, or put some rubbles on it so they don't walk on it. If this is taken as "weakening the challenge", then put an identical, illusory trap to warn them there are traps here, and therefore to reduce the chance of falling in the real one.
Remember that you can be assisted by fellow mages to do all of this. I imagine a 20th level wizard have some old friends to give a hand, too. Or mercenaries :).