Here's what I did when my oldest DS (now 6) went through a similar phase. He would get so angry that he couldn't make good representations of real objects in his pictures. We tried switching to art projects that don't lend themselves to making realistic looking things anyway--we did some torn paper collage, some finger painting, things like that. Those might be good because they could kind of help level the playing field with his big sister--the difference in finger painting is less stark at any age.
This next idea may sound insane, but it helped my son. I introduced him to the idea of abstract art. I understand that it doesn't solve the problem with motor skills and the ability to make things look like real objects. But, with my DS, the idea that 'real' art didn't have to look like something caught his attention. He liked the idea of drawing an abstract concept like a feeling and he loved playing with filling pages with color. It wasn't a 100% solution for us, but it encouraged him to take things in a direction where he could feel successful and still work within the limits of his motor skill development.
