Help please!!

So.. My boyfriend of one month just told me that it's possible he got another girl pregnant ?! They messed around before he met me and now she is late. She took a test and it showed negative but had a faint line. She's only 16 and apparently she lied to him about her age. I'm really happy with him but I don't think I can handle being with him if he was to be a father!! We talked about it but it made me really uncomfortable so I just told him I don't know if I can be with him if she was actually pregnant but he said I will come first no matter what, but I know that is unfair to her and the (possible) baby. Advise please?!