I went on my bfs facebook and found him tlking to other girls?

When i logged on to his facebook ... he didnt know i was going too i saw mesgs from like 3 girls and i opened them one of them had a phone number and it showed that he told her that she was cute ... she looked like a greyhound ... anyways it also said that he wanted to get to know her . then the other girl that he was mesging said that they would have to work out a time that they could hang out because he told her that he worked in albany from 3 until whenever roofing he doesnt roof in albany soo that was a lie and then the third one said that he liked something that this girl had sent him i couldnt find what she sent him he must ahve deleted it .... anyways when i asked him he said that it wasnt him and he swore on his dead moms grave that it wasnt him. But i dont know if i should believe him or not. What should i do?