The wizard may not be allowed to cast spells on his grandson
The rules tell us that, if a creature succeeds on a saving throw, the creature knows that magic was used on it. The grandson always succeeds on a natural 20 saving throw, so there's at least some chance of giving away the game every time the wizard casts a spell on his grandson.
The rules are silent (as far as I can tell) about whether a creature knows when it fails a saving throw. But I would expect that the creature would notice this.
(see: Do you know when you fail a saving throw?)
The wizard may be able to enchant his grandson's equipment
Your question says: "This Wizard can cast spells on his grandson, only." It's not clear if this includes the ability to spellcast on his grandson's equipment. If this does include the ability to spellcast on his grandson's equipment, the wizard can use greater magic weapon and magic vestment on his grandson's equipment the night before the adventure.
The wizard also might be able to enchant his grandson's mundane equipment, for example enchanting his ring into a ring of fire resistance. It's not clear if the grandson would notice the feel of this ring when equipping it.
limited wish can help
You've written that this wizard is "unafraid to spend resources".
The limited wish spell says that it can "Produce any other effect whose power level is in line with the above effects, such as a single creature automatically hitting on its next attack or taking a -7 penalty on its next saving throw." If our wizard has a pile of scrolls of limited wish, he can cause his grandson and friends to automatically hit on one attack per turn. If the wizard has additional buddies to use them, he can cause them to hit on more attacks.
The actual wish spell has an ability which can force a reroll of any event, but unfortunately this offers a saving throw.
None of this improves the grandson's survivability.
The actual correct solution probably involves Bluff
The wizard can improve his Bluff skill to be arbitrarily good. The wager appears to allow this. The wizard can show up (disguised) to the quest, give his grandson a bunch of buff spells and magic items, and convince everyone present that this is normal and does not count as "receiving help". Perhaps the buff spells and magic items would be in a treasure chest which the group can "find". Perhaps the wizard simply explains that the environment in this cave sometimes heals or buffs people and this is normal.
This is better than approaches such as modify memory because they don't allow the adventurers to realize anything unusual is happening in the first place. A strict reading of the wager tells us that it is lost if the grandson "realizes that the Wizard is secretly aiding them" whether his memory is modified later or not. Also, casting modify memory on the three allied adventurers is illegal.
As to what buff spells the wizard should use, the best approach is probably to use greater rings of spell storing to allow the grandson to summon monsters to fight for him. Normally-self-only wizard buffs such as undead anatomy IV can also work.