Ugh...how do I stop this behavior?

Let's see...some background info. Will has a chromosome deletion, significant apraxia, and is somewhat verbal. Meaning one and two word phrases mostly. He has a lot of spectrum-y qualities, and I'm considering looking into pursuing that diagnosis.

We are also looking into ABA for him. (Oh, and he's four.)

So...he's come a long, long way since last year. Once we began treating his metabolic (mitochondrial) deficiencies...he was especially deficient in carnitine....he started to come alive. An awakening of sorts.

However, there are a few behaviors that I don't know how to extinguish, even with ABA. (Oh, and I teach special ed, so I've tried quite a few tricks as well.) For instance, today, he was drawing with crayons at his school when we went to meet his preschool teacher. After drawing some letters and his name, he began rolling the crayons off of the table onto the floor. I can redirect him back to what he's doing, but he seems to go back to this behavior eventually. Definitely a stim...

Any ideas?