My boyfriend cheating on me

Me and my partner have been together just under a year now, when we got together we was just seeing eachother for a while. But I didn’t know he was sleeping with another girl at the same time but I had a gut feeling so I done something I shouldn’t and went down he’s phone to look. We broke a up a few months into the relationship for 2 weeks because he found out that I met up in a coffee shop with my kids dad to discuss my kid. I didn’t let on at this point that I knew about him and this girl. 2 weeks passed and we sorted things out we got back together and I told him I knew about this girl he promised he wouldn’t do it again and how they don’t talk or anything like that. Lately I feel like he’s going back to he’s old ways going out at night for hours, don’t want to even touch me, don’t have sex with me no more, secretive on he’s phone. I logged into hes social media and see messages that he had been meeting up with her he says there’s a reason behind it and how they ain’t had sex but last night he went out for hours today I’ve gone to do a wash load and he’s boxers were all rolled up so I’ve unrolled them to put them in the washing machine and seen 3 white stains in he’s boxers never has he had that before. When ever I confront him about anything he switches it on me and makes me feel like I’m being paranoid what should I do cause I do believe he is cheating on me.