If you want something that you can just buy and use, that will not dry out immediately like a regular lube, you can try Astroglide Gel. Make sure it's not silicone based (Astroglide X, Astroglide Premium) as those are super hard to clean.

Depending on how it work out for your guy and you, if you feel like you're gonna keep experimenting it will be best to invest in something that is more suited for fisting, like K-Lube.

K-Lube is also water-based, it's easy to clean, but it's a lot more slippery and long-lasting than regular jelly lubes.

It comes as a powder and has to be mixed with hot distilled water, after preparing you must leave it cooling in the fridge or freezer. Water must be hot or else it will not dissolve, and tap/drinkable water comes with minerals that can also affect the dissolution so distilled is advised.

K-Lube is the only brand that is made for fisting, most other alternatives are originally intended for veterinary use.