You can't be for sure (well unless you hire a personal detective or follow him around 24/7) but being realistic it is impossible to be 100% sure.
You just have to trust him, thats what being in a relationship is all about,
is he acting perfectly normal? is it just a sense of lingering doubt that a 1% chance he might be doing something else?
Just relax, believe him, trust him & stay sane.
Thats all you can do.
If your really in doubt give him a great time for the next couple of months so you KNOW he wouldn't even consider anyone else.
+ even if its unnecessary your efforts wont be a waste of time because they will only make your relationship better (whereas detectives & stalking him, or even criticising & questioning him about it, could be a total waste of time & even damage your relationship!)
There is only one clear winner here Snoopy73