is he eating hay? has he lost weight? is he eating, moving around, pooing normally?
if he does have teeth problems that are fixable, it will cost you a lot less to take him now than when it gets bad, not to mention what he'll be going through. if he's not chewing because of his teeth it means he's in pain. if you leave it, it will not only be teeth problems you will have to deal with but digestive problems too.
a rescue girl i recently adopted was fed completely wrong diet and as a result ended up with spurs on her teeth as well as a huge (and i mean huge, it's like a tusk!) incisor root. she had about a week to live when i found her, i have spent about a thousand pounds on her in the past four months and i wasn't even paying for the vet bills then! she doesn't need her incisor removed at the moment but if she does in the future, it will cost thousands. if we keep her teeth in check and keep her eating hard stuff, she will hopefully never need this surgery.
a vet visit will not cost you much. mine charges about £14 for consultation. they will be able to check the molars to some extent but they may suggest an xray. these are not too expensive either.
if it's not teeth, great news! if it is, you can tackle the problem early. good luck!