☆.A.
Tell him the truth and that it had nothing to do with him. Would this man still see him? Hang out? So sad. My heart breaks for your child and I don't even know him!
This situation is exactly why I feel strongly that single moms, while free to date, are wise to keep dating completely separate from their young children. Get a sitter--go out. Don't drag the kids through your dating ups and downs.
I'm not trying to be harsh, but it's easy for women to think that their kids are cool with their BF and with them dating and then this happens. Please--your son doesn't need more of these experiences of men leaving him (and that is how he will see this, you know).
Really remember this feeling of your heart breaking for your son's loss.
TRY not to have him put into this position again--until the man is going to be his stepfather.