My ds will turn 8 in October. He also has ASD. He is still harnessed in a Britax Regent (which expires next spring), a Graco Nautilus (but he's almost outgrown the harness in height) and we just recently bought a Britax Frontier 85 for him. He still has quite a bit of growing room in the harness and then it turns into a booster. BUT, the added plus is that you can purchase a SecureGuard piece to use in booster mode. This should help at least a little with the "wiggles" and slouching down when falling asleep. He's in the Regent in my car, the Frontier in dh's car, a Graco Highback Turbobooster as a spare for grandparents (rare occasion) and in his bio-dad's car (he rides in it twice a month for very short distances). The Nautilus is "retired" until fall, when the Frontier will be moved to my car and the Nautilus moved back to dh's car (in booster mode). But his main carseat will remain a harnessed seat for at least another year or so.
I would strongly suggest you find somewhere near you that sells the Frontier and try your ds in it and see how much growing room he has before you make the purchase. Good luck!