My son was also a sleeve sucker, he is now 9 and has been shirt sleeve free for about a year now. With high stress it will come back for a short period of time 2-3 hours in a bad day.It really will end if you remind him to " get that thing out of your mouth" Good Luck
Dear Renee,
You are on the right track in understanding the purpose of your son's behavior. His behavior is a form of self soothing. He will eventually stop this, so you needn't be alarmed. I say this because you point out that he is developing well and appears to be happy. Sometimes the best thing we can do is exercise our patience.