Hi, R.~
I'm a 52 yr old mom to 4 adults. If any of the first ones had sucked their thumb(s), I'd have been mortified; but the more I read, the more I understood that if something comforts a child and/or makes him/her feel more secure, don't try to take it away because that will only make him/her feel MORE insecure/uncomfortable. Our youngest (now 21) was a thumb-sucker -- AVIDLY! When she started pre-school at age 4, she quit. I had told her that it was making her teeth 'stick out (which it was -- she had to have braces), but it had to be HER decision. It's not something that's going to make him unhealthy; it's more of an embarrassment to the parents (right?)
Make it his issue and encourage him to stop before kids start teasing him (I think that's what happened to our girl).
Anyway, it was so cute when she'd go to bed and she'd concertedly try to keep her thumb removed from her mouth, then when she'd be almost asleep it would 'find its' way' to her mouth. She'd immediately jerk it back out! LOL She 'broke' herself w/in about a week or two!
God bless, and I'm sure it'll be fine!