It is definitely your BF's baby no doubt. :)
The baby would be your boyfriends if you are positive that you didnt have sex with your other childs father before hand. Just because baby may have been in its beginning stages does not mean you can alter dna patterns.
ummm your bfs of course if you were already pregnant!
If you had sex with your ex AFTER you got your BFP, then for sure it is your bf's baby. There is no possible way this could be your ex's baby, you were already pregnant when you had sex with him ..
good luck to you!