But, OP, to the other woman - you are just some random person who lifted her baby's photograph from her FB account, I'm afraid. And whether that baby happens to be your "boyfriend"'s child, then I'm afraid it might just be one of many half-siblings, or potential half-siblings to your baby.
As others have said, this boy you're with? He doesn't love you. He doesn't love this other young woman. He might love your baby, he might love her baby (if, indeed, he turns out to be the biological father)... but he loves himself and not practising safe sex even more, I'm afraid. You, your baby... low down on his list of priorities. And you're a mother now. You have to act in a responsible, mature way - and value yourself and your child more than you value this boy who had such little care for either of you that he disappeared for the first few weeks of your son's life... and probably only came back because whilst he doesn't want you, he doesn't want you to be with anyone else, either.
Somewhere along the line, you have to set a good example to your son. Don't plan a future with someone who will continue to treat you and your child like rubbish trailing in his wake. Don't you think you're both worth more than that? Because I do... and I don't even know you.
Yes, he should have a DNA test - but I think he already knows the other woman's baby is his. He just doesn't want to admit to it, because it'll rock his "have his cake and eat it, too" little world (and that'll be two babies he has to fund for at least the next 18/19 years - and I say "at least", because I know parents who still help their children well into their 30s!). And if that baby is your son's half-sibling... be prepared to allow them, when older, the chance to be brothers. Don't poison your child's mind against his half-sibling(s). Just like your child - they didn't ask to have this boy as their biological father, after all. One day, you and this other young woman may even turn out to be quasi-friends, who can laugh at how daft you both are to have wasted time on the boy who seems to think that he has the right to treat the mother(s) of his child(ren) like they're worthless. But be aware - you and she? Will not be the only ones he has children with, unfortunately. For all you know, your child might not even be his firstborn. And if he's not, would you like him to be called "a mistake" by his half-sibling's mother?
Take some time, talk to people, get RL support... and leave this boy who is, by not using protection/to-ing and fro-ing/exerting weird control "I don't want you, but you can't move on" over you, putting your physical and mental health at risk. Your son needs his Mum to be healthy, strong, and responsible. So be that for him. For yourself.
But not for the boy who doesn't give a shit about anyone but himself... and his penis.
