sit him down and tell him your suspicions, say that if he tells the truth you may be able to work round it but you cant go on worrying about it all the time, if he then admits try and find out what has gone wrong with your relationship and then decide what you want to.
A word of warning if he has not cheated and you keep accusing him then chances are he will do it, mens logic - if I am going to be accused all the time then I might as well do it, So talk sensibly to him. Good luck I hope it all works out for you.