I don't know if it's necessarily the right thing to do, but if your boyfriend is always talking to his ex, then I would be suspicious. It's hard to trust anyone these days unfortunately. As someone who's been cheated on, has cheated and was with someone who was cheating on his wife with me, I'll tell you that if he has an intention to cheat, then nothing will stop him. People will go to greath lengths to get away with it, and cover it up.
You could sit down and have talk with him about the fact that him talking to his ex makes you uncomfortable. But if he really still wants to talk to her, he will. He'd just be more clever at covering it up later on. Do you know where his ex lives? I'd say you might as well keep on tracking him, if it'll give you peace of mind. Anyone who says you should trust him completely is a sucker.