What should I do?

[esmerelda]

So you have been TALKING TO this boy for 3 months. Am I correct in surmising that you have never actually MET him?

You haven't been 'dating.' You're pen pals, nothing more.

He's not "cheating" if he talks to someone else, because you're not dating. Dating is when two people meet and go out together (or stay in) and DO something TOGETHER. You can't "stay with him" because you've never BEEN "WITH" him. You're 100 miles away and he most likely has an actual, real-life girlfriend who isn't aware of his cyber-sex games.

This guy sounds like a player, who is gaming Snapchat to see how many girls he can virtually hook up with.

"Is there a way I can trick him or manipulate him into telling the truth?"

The only thing I can suggest is to break off contact with this conman and don't be so gullible in the future. He is NOT your boyfriend. He's a guy who likes to play sexual mind games on the Internet. And you are one of the suckers he hooked.