No, this is in no way a way back in with you. Why would you think it was? Do you perhaps still have feelings for this man?
Do be careful please. I know people here are saying it's positive and should be encouraged, and on some level I do agree with that. You don't want him turning round to his new gf and saying 'see, she makes it impossible, this is why i've not seen my child...'. Because he will be trying to make it look like he's the good guy here.
But it does certainly sound like this has all come about because of the new gf. As a single parent herself she probably wouldn't want someone who can walk away from their own child. She's more than likely questioning his role as a father and encouraging him to try again (he's no doibt said how impossible it all was trying to remain civil with you). And he's taken that on board becase he's trying to impress her. So this is mostly about her and unfortunately not very much to do with your ds.
So your ds is in the middle here and isn't really the reason for his change of heart. I don't think people change that dramatically, and certainly not overnight. Is he paying child support? If he isn't, or isn't paying enough now would be the time to raise that issue, so he can prove that he's genuine in wanting to accept his responsibilities to his child, and the gf will not let him wriggle out of it (having been there herself most likely).
But before you go ahead with all this please ask yourself some important questions....
What will happen to your ds when his dad decides he can't play happy families any more? Or he sees the new gf's child gets all the time and attention he doesn't get from his own dad, how will he feel then!? And what happens when he turns around and drops him like a hot potato if the relationship ends. Or he starts phoning you to blame you for any behaviour ohe or the gf doesn't approve of when perhaps your ds is trying to get his attention, or is reacting to teh situation, or just doesn't get the new gf's rules, or your ex finds he can't deal with the invasion of his space and time simply by your ds being there...
Please tread carefully because this has the potential to seriously affect your ds's self esteem and confidence if his dad lets him down big time here. Does your ex understand that. Does he really understand that.
So ask your ex what it is he is proposing. You could even say you believe this change of heart is becuase of the new gf and that while that is a good thing you need reassurance that he will maintain the relationship with ds regardless of the new gf, not simply because of it.
Ask for confirmation that he will spend time getting to know ds and time together, not simply be invited to keep the gf's own child company.
He's probably going to say the right things now anyhow, whether he means them or not lets hope you never have to find out.
I think you need to be aware that there are potential pitfalls here for your ds and be ready to listen to what he wants and support him through this. At 10 he will clearly see that he has been ignored all these years while this new child gets to spend time with his dad and that could raise issues for him depending on the level of input that other child is now getting from him.
But yes you must try for your ds's sake, if that is what he wants to do. Just be ready to deal with it all when/if it all falls apart.