I have a lot of big time dental issues, all genetic. I never sucked my thumb but have the same issues as my mother and grandmother.
My daughter sucked her thumb and at her 1st dentist appt (at 2.5 yrs old) the dentist said she could see how her mouth was developing in an undesirable shape...not the teeth, the whole mouth is effected, the palate and the jaw muscles. The dentist told me that if I could get her to stop then, she should outgrow some of the "damage." Then she had a growth spurt that changed the shape of her face and the way she pronounced the letter S. Given the genetics in my family, I wasn't gonna fool around, she was going to stop.
I used the yucky tasting nail polish. I painted her thumbs when she was sleeping because I was afraid that if I did it when she was awake she'd also think I could un-do it and that might lead to conflict between her and I that I thought would be counterproductive. One morning she woke up and her thumb didn't taste good anymore. I hugged her and let her feel sad about it.
I got it at the local drug store for $5-ish. The instructions say to paint it on weekly, but I only had to do it 1x and she was done.
Thumb sucking is self soothing, so he should be offered an alternative. Mine has a beloved blankie that she loved long before I forced the thumb issue. At the same time though, if he had a pacifier they probably would have taken it away by now too, and he'd be fine.