Sucking on thumb

I've only been running a daycare for about 1.5 years, so I've really only transitioned one set of one year olds. I recently started a new 15 month old, and he constantly had his thumb in his mouth. He's gotten much better now that I've had him (it's been about 6 week I think), but as soon as mom picks him up, his thumb is in his mouth instantly. I think it's safe to assume he sucks his thumb most of the time when he's at home.
I'm curious if there's a connection to this and his lack of ability to chew and swallow food and drink. He's made a tiny bit of progress on keeping food in his mouth when he's chewing, and he still has an awful time keeping his milk in his mouth when he's drinking. What is everyone's thoughts on this? If there's an actual connection, and not just one I've imagined, I'll talk to the mom about keeping his thumb out of his mouth at home too.
I should add this child is behind on a lot of other skills too, so his eating skills could just be a part of whatever it is holding him back in the other areas too.

I doubt it's connected in a physical sense but I would not be surprised if the eating is more to do with how much independence is encouraged at home. If mom is trying to keep him a "baby" she may also be feeding him and if it's their first child they may be a little slower at introducing new things. My 4 yo still sucks her thumb when she's tired or while sleeping. If it is in her mouth during the day she is just reminded not to suck her thumb. It is a comfort thing for her.

Thumb in mouth when he see's mom could just be part of the transition which can be difficult for kids. Who knows, maybe they don't encourage the thumb at home at all.

Initially I thought maybe they were just doing everything for him. But the mom has expressed concern as well regarding how behind he is. And it certainly looks like they're trying. He wouldn't even go down the one step into my toy room. I thought he just wanted to watch the other kids at first, until he was comfortable. Then I realized he didn't want to go down the step. So I asked his mom how he was at home with going from one level to another. They tried it at a grandparents house and he had a huge tantrum when they were practicing stairs.
Everything I noticed about his eating, his mom brought up before I had a chance to say anything to her. I just don't know how to help this one adjust and "catch up" to a normal level. Typically I see little ones change very quickly when they see older kids running and playing and climbing. They want to do all these things too.

I also see them out around town once in a while. They seem comfortable with his thumb in his mouth constantly. I discourage soothers and fingers /thumbs in the mouth unless it's nap time. It hasn't really been a problem until now.

I guess when I think about it more, the problem I have is that mom and I both want him to be able to do things independently, like get around the house, eat and drink, etc. And I have no idea what to suggest to this mom. Initially I just reassured her that he would probably catch up quickly after seeing other kids do these things. Now I'm not so sure.

I don't think it would be connected since they say that straw cups are better for kids to learn how to chew, swallow, speak etc and thumb sucking would have the same affect as a straw cup lol.

I would let mom know that I am discouraging the thumb sucking as much as possible at daycare. I think the rest will come in time. Children develop at different ages so try not to compare him to the others too much just quite yet

I had one that sucked her thumb while she was sleeping to the point that she had red oozing sores above the knuckle on both thumbs .... I pointed it out to the parents that I thought her thumbs were infected ... They took her to the dr and he put her on antibiotics and told them to put mittens on her when she went to bed to break the habit .... So that's what we both did and she no longer sucks her thumb

How did you stop her from taking the mittens off?

I told her to leave them on. if she took them off I put them back on and said "leave them on" with my serious face.....

Thanks. I'll keep that advise handy.