Most artists learn their craft by practicing on their own, getting good, and then taking formal instruction. Sure, a degree helps, but from what I know from the little bit of art instruction I've had, the mother most likely would have been able to teach your character those basic principles on her own.
"Jack, you have debauched my sloth."
Okay, I just wanted to make sure it wouldn't push everyone's Willing Suspensionof Belief , plus the fact if it gets too ridiculous it gets pretty close to a Common Mary Sue Traits.
I think the biggest issue here, I feel, would be the initial Fridge Logic of how the character and his mother were initially able to afford painting as a hobby in the first place since paint colours are essentially dyes and were fairly expensive prior to synthetics being developed. A possible solution to that, if this is enough to give cause to worry, would be to have the character stick to drawing since ink and charcoal - while precious commodities - would probably be far cheaper and easier to procure.
Perhaps they mix their own from raw materials: fluids taken from plants, minerals ground from rocks, pigments taken from insect shells, etc.
My Games & Writing
So one of my characters is an artist by profession. He was raised by an artistic mother who drew and painted as a hobby and she is the one who taught and instilled the interest in him. He never recieved any formal artistic education (he couldn't afford it, and very few people in his setting can) but improved mostly by practice and repetition. His choice of profession isn't entirely impractical (no offense to anyone who is a professional artist, but the 'starving artist' cliche exists for a reason) because his setting has not yet developed any form of photography yet and there is also a large upper class who can afford to buy art. If he has been going out and practicing 'art' for many years (at least a couple decades) would it be plausible for him to be very skilled at it, despite lacking formal artistic instruction. I'm sure it's 'possible' for some people to do that, but I don't want it to be ridiculous.