Most Helpful Guy
I hope not. It's hard to say from just a guy acting weird. There are a lot of things that can make a guy act weird, especially young guys. The fact that he has in the past is cause for concern, but isn't enough proof that something is going on this time.
You'll probably want to be cautious, to pay attention to what's going on, and to me more careful with your feelings than you normally are for the next while and try to think about how he is acting and how it makes you feel. The thing is, even if he is not cheating, if he is starting to act in a way that makes you feel badly, it may not be the right relationship. A guy acting weird can often be a sign that he is not super into the girl. But again, it gets tricky in that mid-teen range because guys are so unpredictable. The hormones they are dealing with really makes it hard for them to know what they want, and they can end up changing their mind really frequently.
I hope things work out for you, but I would be cautious and attentive for the next while and be mindful of how you feel.
Most Helpful Girl
If he cheats once then he will cheat again. Find someone else that will be willing to be with you without cheating. And technically I'm pretty sure we both know that you don't trust him and think that he might be cheating because if you actually trusted him, you wouldn't have asked on here.