Property

I think you need to step back and realize they are his grandparents and he could argue that you did give it to him as a gift.  Why would you of wanted a picture of his grandparents prior to your marriage??  Trust that he will do the right thing and of course pass it to your son.  He certainly would not throw it away...  Good Luck!

I'm a family law attorney in CA but I can help you to better understand what "marital property" is

marital property is assets - assets are tangible items purchased during your marriage, furniture, cars, boat etc - these are the items the court concerns itself with ensuring equitable distribution.

Sentimental items, such as your art, are not subject to the same distribution as "marital assets" because they weren't purchased for mutual use - it was just your creative expression. These items are left for the two of you to hash out on your own in mediation, and the court will only get involved if the two of you end up at a total stalemate about it.

there isn't a judge in the united states that is going to force you to give him art you created unless he can specifically prove that what you painted you also gifted to him.

Only exception I can think of is if any of the paintings he took were images of himself - I can see a judge taking those as an exception and not making him return them.

They are not income generating, and my daughter
has provided an affidavit that they were painted prior
to our marriage. In my 25 years of marriage, I sold 2
paintings. I paint for myself. I am curious, however,
what your background is and if the law is different
in each state. . . for example, north carolina.

Are the paintings dated? If so, proving they were painted before you were married will be easy and they will have to be returned.

If they are marital property or not largely depends on if you use your art for income, or just personal satisfaction. If they aren't income generating, they are yours and he has no right to take them. If they are income generating, the ones painted while married could be considered marital property, depending on how strict on that point your judge is.