I wonder if it is just habit now or he is self soothing. Going to the dentist sounds like a great idea. I used to role play dentists when mine were little. We practiced opening mouths and I would count their teeth. Then vice versa. Helped them know some of what to expect and not make it as scary.

Lol Mom462038, I can imagine that worked but not something we can wish to happen to our kids

My son was a thumb sucker. One day he was playing with tongs, cut his thumb so it was too sore to suck. Habit broken

Let the boy stim.

I’d not make a big deal of it as actual thumb sucking. Maybe a general comment about taking your fingers out of your mouth while out and about.

We told anyone that commented on the thumb sucking to mind their manners and keep their comments to themselves.

The more attention you give it, the more you reinforce the behaviour.
You might find this an interesting read
https://www.webmd.com/parenting/features/9-ways-to-wean-a-child-off-thumb-sucking

This eventually stopped with one of our children after many years, with no need for intervention.

The more you make an issue of this the harder it will be for your child to stop. Once he is at school he will soon stop because no other children his age will be sucking their thumbs.

All 3 of my kids sucked their thumb. I had no issue with it. My eldest two had perfect teeth and my youngest had buck teeth but I think it had zero to do with thumb sucking and everything to go with genetics. I had buck teeth and so did my Mum ana neither of us were thumb suckers.
I didnt ever try to get them to stop as it was a comfort thing for them. Total strangers felt they had the right to comment and I once had a woman tell my daughter not to suck her thumb because it was dirty. My response was “Excuse me, please dont tell her that her thumb is dirty….its not but your comments to her are rude”…the woman was very offended and stormed off in a huff. Im really not sure why she felt it was ok for her to say something but not for me to respond.
All 3 of my kids stopped sucking their thumbs by age 8 on their own with zero pressure from me. I think the more drama you make of it the worse it nakes it for the child.
Look at the reasons why you want your child to stop. Is it because you are worried about what others might think? If thats the reason then thats not fair on your child…Ignore others.