What to do- dont trust him at all ?

been with my boyfreind about 2 1/2 years. the other day he lied to me about a number saying it was his brothers friend or something -- i found out it was a female friend. i also found some fb messages to her- i know im such a nosy gf- but w/e ...she told him that she was pregnant and the guy was leaving her...and my bf was saying things to her like "oh why couldnt i be the one to get you pregnant- i would never leave you ." like wtf.??? who even says that... anyway i told him i saw all the msgs and w/e and he says i have nothing to worry about shes just a friend blah blah blah blah..and he loves me. but i dont trust him. i told him he should have just told me who the number belonged to in the first place. anyway any thoughts or reponses appreciated... i feel so sick about this. im pretty sure im just gonna break up with him. what do u think. thanks in advance.