It can be done, especially since your split was amicable and long enough ago for you both to have moved on. We have talked about this before.
First things first. You are 1000% sure that neither you nor he have any lingering romantic interest in each other? I don't mean affection, since friends are fond of each other, but the love/lust end of the emotional spectrum. If you have, or he obviously has, GTFO.
If not, and somewhat unfairly to you, being friends with him all depends on his new girl now. If she doesn't like the idea, and she probably won't, GTFO.
That is only fair on her chances of a good relationship with this guy. The only way to find out about that is to ask her, which is his job, not yours. I recall a huge and slightly bitter battle in the comments of one of the questions (Can't find it just now) about it being "demeaning" to have to ask permission to have a specific friend.
It is not. It is his responsibility to make a go of his relationship, which means his girl's feelings come first, even ahead of his own.
If she is cool with it, then keep it light. The odd call or email, maybe the occasional meet-up for lunch or a drink or fishing or whatever interest you share. Nothing at all "date-like" though, that sends totally the wrong message.
There are a mc’zillion people out there…, go make new friends. Can you imagine how insecure his gf would feel with you lingering around? I didn’t like it & I’m a very confident person.
What's the point of this friendship really??? I mean, if you're only going to see the "friend" once in a while, and keep the conversation to only specific topics (bc usually you don't want to talk about the people you are currently dating), what's the point? I don't know, sometimes I think it's just better to move on and like Silkysly said, make new friends.