The article focuses on this issue when thumb-sucking begins to affect secondary teeth, as hammock and the article point out it becomes an issue when the behavior is retained with secondary teeth. 3.5 years seems like he's reaching the threshold for when to ween.

"There can be downsides to thumb sucking, as it can lead to problems from pushing secondary teeth as they come in. But those don’t come in until later, when children are in elementary school. Most children stop sucking their thumb by themselves before they get to elementary school. The American Dental Association recommends that parents start encouraging their children to stop sucking their thumb after age 4 — but even then, they advocate a gentle, positive approach. Many children suck to relieve anxiety, and making them anxious about sucking can backfire! Instead, praise children for not doing it — and help them find other ways of soothing themselves. Putting a Band-Aid on the thumb can help them remember; at night, using a sock or something similar can help with the child who sucks their thumb without realizing it. But again, this is only an issue for older children."