Should I ask him if he's cheating?

Well,, my boyfriend has been having weird screen names on msn saying ily kristie, ily katie, single :( and stuff likee that he dont no I’ve seen them. Andd I was going through his old posts when I seen these andd I think he may be cheating. I was suppost to ring him today because we hadnt spoke for agess or seen each other and I texted him to ask him if I could ring him now. I didnt get a reply for a while and when I did he said but im at my cousins 18th babe so I went just forget it then. But the thing is he’s 18 and im 15 and he wants a baby and so do I but because we dont really see each other much its pretty hard but im moving closer to him if I do end up pregnant. But the thing is I dont no if he’s cheating or no what to do. I dont no whether to ask him about it or what. Please help thnx x