can I please get advice !?

Jenny • California • 17
What should I do , what would you do??? 
Okay so I found out my bf has been talking to his ex behind my back & he admitted it but said that he never went to visit her. (She still wants something with him although she's a single mother age 16) & well I'm so upset he said he's gonna change & be serious about us now & I told him it's kinda too late but that I'll have to think about it. Should I give him another chance or let him go?? (We have 3 years)