Ask the one whose heart won't be broken by the question, i.e., your ex. He knows that you slept together pretty close to the time when you got together with the new guy. Your present guy doesn't know this, I guess, if it would break his heart if he's not the dad. So, get you, the boy and the ex swabbed and send it off. If it's not a match, then you know your present guy is the dad without having to break it to him that he might not be. If it is a match, then you have some telling to do.
Thank's for your comment. Yes I totally agree that it's only fair to my son and his father. It's just so hard to build up the courage to ask one of them for a dna sample because it's been so long. But I can't keep living in denial so I am going to pluck up the courage to do something about it.
You won't ever know by guessing. Dates are too close. You HAVE to get a DNA test. It's only fair to your son to know who his true father is, and for the father to know as well.