couple things... why are you posting this here? You would probably get better advice in /r/relationships. This sub is mostly for men not getting laid by their long time spouses often times kids are in the mix. They are cynical and jaded and all kinds of resentful.
It sounds like you are in need of actual therapy, not as an insult but the kinds of mental and emotional knots you have displayed need a professional. You have caught yourself in an anxiety loop whereby your anxiety about him cheating has seemingly caused you to pull away from your relationship and not be an active participant in it. This very act is likely to cause or increase the likelihood he will look for affirmation and even physical comfort outside of your relationship, thus providing you with fresh anxiety.
If you two were on a break in the summer of 2014. You are looking for proof that he did something during this break? This is not born out of a desire to root out infidelity. This is insecurity and you are looking for justification to feel insecure and unloved. As we have taught our young folk that romantic and sexual love are this completely intertwined and irrevocable thing pumped out of Disney factories. The thing is people cheat for a lot of reasons most of them are due to their own insecurities. In fact based on your line of thinking I'm willing to bet you've had a few totally understandable and completely minimized indiscretions of your own. Don't fixate on that statement if it's incorrect. just an asserted possibility.
You significant other may be cheating but it's almost always a precursor to leaving a relationship. Insecure people don't end relationships because they aren't working. They find someone new that makes them feel temporarily good about themselves as they are awash with all those fun neurochemicals that make you pop and fizz. definitely at various points in my life I would catch and release at bars, chat up chicks and flirt but as a way to feel better and more confident. That is until I found better ways of dealing with my insecurities, part of which included leaving a relationship which heightened them.
if you want things to get better you gotta get right with yourself you need to truly heal from what happened to you before. you need to learn how to have faith in your own worth and value not as represented by your partner but in your own self posession and identity. You have to risk loving someone fully with the knowledge you may be hurt, otherwise you definitely will be hurt.
Think about it like this everyday you are given a little magical heart. in an ideal relationship you give the majority of this magical heart to your significant other and they do the same for you. You get more hearts. now when someone is cheating they are getting the heart from you but taking a bit of theirs and giving it to someone else in exchange for some share of that persons heart. What you have admitted to doing in your relationship is withholding your heart from your SO. he's getting nothing from you anymore. So it sounds like he's probably flirting and chatting up girls and only he knows what else. but the thing is why shouldn't he? He's not getting any magical heart from you. At that point it's not cheating, it's looking for a better deal.