Advice...what would you do

So the other day me and my SO was on the way home from Maryland and his phone rung . His phone is hooked up to the blue tooth in the car so i can hear the conversation. Its some girl who ask him is he off work yet and do he have money(not much $10) but still. So i ask him who the fuck is she. To me he paused before giving me what seem to me like a BS answer and she hung up before i can ask her any questions. She called from an unsaved number. The next day i told him i wanted to talk to her he said ok i guess he thought i was bluffing because after i grab his phone he was like nah don't do that blah blah blah so i said ok. The next day he came home late from work (he usually calls)so me being worried i called him...no answer...i called again and got a text saying call you in a minute. When he called me i got what i felt was another bs answer. Our relationship hasn't been the best since we had our son. He is 3 months. We went from being bestfriends doing everything together, to fussing and him hanging out with his friends. Yesterday we decided to take a break. I told him i felt like he was cheating and i was tired of begging for time i didn't use to have to beg for. We still live together he told me i can stay as long as i need to (its technology his apartment). He claims he love me and he was not cheating but instead of trying to work on us if he really want to get back together he tells me he going partying this weekend. I lost my bestfriend, im lonely and i really just don't know if i should just give up and be the best co-parents we can or try to fight. He really is not a bad guy.