I see you are expecting. Congrats!
Do you think Ian is picking up on the idea of the new baby and might be using this habit as a comfort/stress relief?
I would suggest that you give him something that is "appropriate" to chew/suck on. I am not saying that your son does, but my son had an oral muscle dysfunction. He still drools sometimes at almost 8, but he used to chew on rubber toys to strengthen his muscles as well as sooth that urge to chew. This was suggested by his speech therapist.
If Ian is biting his shirt, a good alternative might be a small blanket that he can bite. He is old enough to understand if you explain to him that if he wants to bite, he can only bite the "bite blanket". This way, if it is an inappropriate time, you can take it away for later. Then you can be sure to keep it at home when school starts in the fall.
The only other thing I would suggest is to check his teeth for loose ones or even new molars growing in. Sometimes they bother enough to want to bite or suck.
I hope this helps.
P. <><