Is my boyfriend cheating on me with his ex?

ok so meand my b/f have been together for 2 mounths and ever sence I been wit him his ex has been callin his phone and she always says I am his baby mama cause he got her pregnant before he was wit me but she got an abortion and he tells me all the time that she is a slut and that he hates her gutes because she killed his kid but yesterday he went to jail on my b-day at that and his brother was holding his cell phone 4 him intill eh gets out

bu I called his cell and his sister had it but then is ex girl got on the phone and she was running her mouth and she told me that her and myb/f we still haveing s e x but I don’t know what to think cause she has been trying to get wit him ever sence we got to gether so how do find out if he is really cheating on me wit her I know wait intill he ets out but how do I know if he is tellin the truth or not? I need help I really love him but I don’t know what to think any morei trust him but what is she is tellin the truth?