Dear OP - it's difficult but just no. You've had several great pieces of advice on here and Lynn's post is just amazing. As a PP said, it should be required reading.
The enormity of the cheating is at least part of the issue here. This wasn't a drunken kiss with a colleague or even a drunken one night stand with a stranger from a bar. That could be problematic enough. He had a whole other life with this woman. He went on holiday with her. As PPs have said: how much lying and deceit did he need to practise in order to carry that off. What do you think he was telling her about you and your child? What lies do you think he was spinning in order for her to think it was a good idea to go on holiday with a family man? Telling her you and he didn't have sex any more, he didn't love you, he was only with you because of your shared child. Imagine all the time, money and emotional energy he put into this relationship with her which he should have been giving to you and your child. He cried when he got found out. Boo hoo. He didn't cry at the thought of what he did to you while he was doing it, did he, while he was getting his end away with the OW? This man only cares about himself.
All men do not cheat any more than all women cheat. My understanding is that if you look at the stats, the genders are about even on cheating. Your friends are just saying that because they made the decision that seemed the most immediately comfortable to them. And perhaps their partners had not actually holidayed with other women.
Of course, your ex looks irresistible now. It's been a very difficult year for single people and he's on his best behaviour. No doubt he has some superficially good qualities and is very physically attractive to you. As a PP said - you are, in a way, in the honeymoon period with him. You're not going through the daily grind together. But look what he did when you were going through the daily grind. As PP have said - how will you feel every time he talks to another woman, every time he checks his texts, every time he's inexplicably absent somewhere? Getting on amazingly didn't stop him creating a life with another woman first time. There are men out there who will treat you well and be loyal. And who don't only care about themselves.
Good luck with your decision, OP.