It's a long term problem I haven't managed to solve myself. I have two very different perfectionist.
My eldest is the push, push, push until you are perfect sort. That internal drive to be perfect is so strong. Even she as a teenager KNOWS that it's unrealistic but admits, it's an impulse that she can't always control. When she was little, we did what you are already doing.... praising effort instead of performance. Modeled positive reactions to imperfection with my own doings. We steered her to activities that focused on "personal best" and required long-term study. We verbalized what she was doing.... "I know you aren't happy with the horse head you drew but personally, I really liked how you made the mane flow. Art is all about how it makes a person feel and your horse made me feel really good." Ongoing acknowledgement and talk has seemed the most beneficial but like I said, she's 14 and it still rears it's ugly head. It's just better than it was and I'm starting to think that is all we can really do.
My youngest is also a perfectionist but a very different sort. He doesn't want to try for fear of not being perfect. He does not push himself to do his best. He does "enough" and likes to be able to say "I did it fastest, I did it without trying." This gives him an excuse for not being as perfect as he really feels he should be. We had to make sure he was in an appropriate academic situation that expected him to work above level as opposed to an environment that gave him options (because he would not take the options.) I'd be OK with him on a slower path if HE was OK with it but he's not. Nudging him to take challenge, risk trying hard and possibly not succeeding always ends up with his feeling really good about himself no matter the out come.
I'd stay positive with your son. Take him to see some famous art and point out how lots of art isn't representative of real life. Show him art that you might really love but isn't perfect. For example, I really like Jackson Pollock. Yes, it's just paint spattering but when I sit in a room with one of those giant canvasses, I can't help it but my imagination is sparked. Acknowledge what he's feeling out loud. Give him the vocabulary he needs to express those feeling. Otherwise, keep doing that you are doing.