This did not happen to us personally, but we have friends whose son was seeing an OT for upper body coordination issues too. He did not show the intense sucking and chewing need that your son does -- but the OT and his parents realized he could concentrate much better if he chewed at the same time he did other things, including schoolwork. As part of his written "individualized education plan" with the school, the OT recommended that he be allowed to chew in class, and the school had to say yes since it was an OT recommendation. Rather than chewing gum, which would have made every other kid go nuts with desire to do the same, he chewed specially made items (his mom called them "chewies") that OT therapists can provide. Ask the OT about whether it might help your son to chew something acceptable for chewing whenever he feels the urge to chew, bite and suck other things. And if he does not already have a written (and enforceable!) individual educational agreement with school (which means they have to accommodate him in certain ways), talk to the school about how to get one drafted ASAP.
If he's getting his OT services through school, also, consider whether he might need additional OT or other services outside school and more often.
One last thing -- I'm not sure how to handle chewing the bedsheets which obviously is at night. He could not have a "chewie" then because of course he could fall asleep with it in his mouth. The OT will have ideas.