i don't think boys like to be asked such questions...but you need to have a proof or if you think something is really not going right,confirm these things as well-
ask for his phone,if he is hiding something,if he talks less or ignoring you..
then you may just ask why is he doing this or if he want to end the relationship...
Well first of all, you must have a reason for why you think he is cheating on you....
You should not confront him unless you have high doubts about his faithfulness.
Be as firm as possible and remember not to let any man play you as a fool.
As what they all said, Make sure you have the proof and his behaviors have changed. If you come out and ask him with the evidence of the affair is will damage the bond of trust between you two. Guys will let it go, but they'll never forget it.
I kinda like finding out the truth first...if it's true, THEN ask. You'll find out if he's a liar, too.