Actually, have you checked to see if his shoulders are actually above the straps where they come out of the seat back? I found that on my XTSL, it "looked" like it was "outgrown" (ie, shoulders above top slots) almost 1/4-1/2" before it actually was, because the bottom part of the wings and the cover sit just so, even in the highest setting, that it was making it look like the straps came out of the seat back lower than they actually were. My DS1 looks pretty close to what your DS is, and I thought he had grown over the slots...but when I actually checked where they actually came out, they were over his shoulders still. He didnt like the feel of that, so he's now happily in an 80SL with plenty of room above his shoulders still.