Should I or shouldn't I?!

I've posted on here a few times in the last month about my situation. Basically my partner and father of my son split with me around 5 weeks ago because he no longer was in love with me. We'd had problems for a few months, but I never expected him to walk away. Since then, he's been really down and upset, and said he 'always makes the wrong decision'. I haven't pressed him for answers because for the first 2/3 weeks or so I questioned him until he got fed up with me. He has said a couple of weeks ago, he'd made the right decision because 'we just wouldnt work'. But yet with him being so miserable I don't know if he really thinks that and I told him even if he wanted to come back I wouldnt have him.

Each day that passes I am feeling better and better and I'm realising that things weren't right between us and that maybe this was a blessing in disguise. But now I have a problem, out of the blue, without even planning it, I have met someone else. It seems so soon after the break up, but it just happened. We get on so well and he makes me smile. But I'm not sure if this is right. Or if i'm on the 'rebound'? Is it fair on the man who I now like? He knows my situation and is willing to go along with things for the time being as friends, but my son's father would be so hurt if he knew (I would too if it was the other way around). He also still lives under the same roof as me until the end of September. Am I being cruel doing this? Any advice would be helpful please.