Hi! Glad you're here!
It could be perfectionism. But sometimes kids that age want to learn from you how you make things. DS7 used to ask us to draw trucks, so sometimes we did. As long as he was watching what I did, I figured he was getting something out of it. Pretty soon, he was drawing trucks better than I did. (At 2! No joke! Of course, I'm no artist...)
Maybe you can try taking turns? You paint something he asks you to make, then you ask him to paint something? Talk about what you're doing: "Now I'll make the roof. What shape should it be? Rounded or pointy?" That sort of thing.
And sometimes say, "No, I'm not painting today. It's your day to paint" or something like that. That way you're not a crutch for him. Mixing it up is good, I think.
FWIW...
