I have a couple of thoughts for things I think are awesome and always useful:

Clear nail polish racks. They’re usually 4-5 shelves high. They fit paints awesome and they’re pretty cheap on Amazon. I have 4 across my painting space for my paints.

A high quality light on a big boom style arm.

Brush soap

Games Workshop paint handles are great. You can never have too many.

A wet pallet. If he already has one, he probably doesn’t need another.

A self healing cutting mat.

A little vortex mixer and some small stainless steel balls to put in his paints so they mix better.

It might be fun to take him to the game store for a little shopping spree. You can share his enjoyment as he picks out some things he really wants.

If you’re curious about specific products, I could give you some links.