How old is your ex's mom? And how old is the new guy? That sounds... creepy.

Honey, the boy is not a piece of meat or a trophy. He's is a thinking human being with the ability to choose who he wants to be with.

As for what's socially acceptable, AHEM:

Do not date your friend's ex unless they ended on good terms and you are certain that your friend has no more interest. Be sure to check with your friend about it first.

How this rule applies to you:
He's YOUR ex. You get first dibs on getting back together. Mom is the friend who needs to get permission first. New guy is the friend of your ex. Now, guy rules are different, but it's up to him if he decides to risk it and go for the relationship with you. That's his choice, not yours. You are in the social clear.

If it still bothers you, talk to the mom about it. Tell her how you feel, and that you really want to get back together with him. You want her to be ok with your relationship, and you don't want to cause strain on the relationship. If she's a true friend she'll understand and everything will be cool.

In the end, you have to decide if this new relationship is worth the problems it could cause. If it is, then go for it. If it isn't, then don't.