Is he cheating... :/

So me and my bf have been together for 3 1/2 yrs. We've known eachother our whole lives tho, we grew up together and went to the same schools. 6 mos. Into our relationship he proposed and I was the happiest girl in the world. Everything was going great until last yr. I caught him talking to his ex on Facebook about what they had and how he missed it sometimes and they were reminiscing about the past. Well she had made some comments about her wanting to go over to our apartment while I was at work. So after I read that I was upset and I went top my grandmother's house for a few days. After I left him because I didn't trust him anymore and I found out he had stolen $1400.00 from me the apartment was still in my name and everything in there was mine or belonged to my family furniture wise. While we were separated he would pretend tip be strangers online tip see if I would go and have sex with them when I'm not that type of person and I was still hurting. He would text me everyday still and say I'm sorry plz I love you I miss you stuff like that. After a month I found out that he had met another girl online and took her up to the apartment and had sex with her in my bed. Ask while still trying to get me to take him back. All in all I ended up taking him back and so far it's been ok. He promised to pay me back and he paid about $600.00 of it. But lately I've noticed some things that make me worry that he's cheating. He has a password on his phone now but I know the code. He deletes all of his messages before he comes over and gets annoyed when I ask for his phone. He has a Windows phone so it can store allot of messages and don't need to keep things deleted constantly. And now I've noticed that he has some other girls on his Facebook who I don't know who they are and we didn't go to school with. Every time I bring up the subject and ask him what's going on he says he's learned his lesson and don't wanna lose me again. But I have this nagging feeling that he's talking to these other girls and he knows that if I find out he is then I'm leaving him for good. I would put a keylogger on my laptop but he don't check his Facebook on it anymore. He will only use his cell. I just don't know what to do anymore. I don't feel like I can trust him and I'm worried that he's cheating. :( if you've read all this thank you for taking the time to do so lol I know it was a long post. I just needed to get it off my chest. Maybe some of you ladies have some ideas into this?