The statement "I miss you already" is one you make when you have "just left someone's company". It isn't the kind of thing that is said when you hang up the phone. His reply is the answer to your question. If you are in a committed two year relationship and living together, he should not be spending time with his ex even if he is not sleeping with her. If he isn't sleeping with her now, it is only a matter of time, in my opinion.
Whether he is sleeping with her or spending time, he is cheating on you because you are unaware that anyone else is even in the picture, until now.
If he was going to be honest about things he would have sat down and discussed his feelings before he chose to wander.
I don't think discussing it with him will give you the truth but it might be worth a try.
However, I am assuming if you are living together you are doing the trustful things a couple do like mingle bills and finances and share cars etc.
I would make sure that I was not in a more vulnerable position than you already appear to be in. I know the emotional side is more important to you right now but there are some things you might consider.
If he is cheating he might not have felt it was convenient to tell you for many reasons.
He might not want to hurt you, in which case he should have talked to you long ago.
He might not be financially well off enough to leave.
If you confront him without making sure that any mingled accounts are secure or that you have transportation if you share a car, you might find yourself surprized by the outcome. Take care of your situation and be prepared that the outcome of such a confrontation may well reveal that you should part company only on your terms.
If he again denies any wrongdoing, I truly feel that you would be foolish to believe him and not make plans of your own. I realize this may sound crass and material but take it from someone who has been there. The pain is only made worse when you realize that not only did they leave you but they emptied the checking account, took the only car and walked off with the T.V.
