I would not recommend it as only few people can make this situation work. Chances are very slim. You have to be extremely patient and smart and he has to be extremely wise and cold blooded.
It is not suitable saying that engaging into a relationship with a married man is a bad decision on its own, but be ready that:

His child will always hate you and have trauma of loosing his/her father for the rest of his/her life, blaiming you on his parents separation. Even if it's not completely true. Wait till they divorce and avoid any relathioship till then- I think that what you have done, right?
Secondly his child will hate your children and feel very jealous if you manage to stay together with your to-be husband. Be prepared.
Be ready that his (ex/other)wife will make you mad from time to time as well with her demands and scandals. She might have every rights for that, so don't be surprised.

However, on the other hand we all humans and man as weaker type of humans can get attracted to other woman due to different reasons. Try to understand why he is with you. It will help you later to hold him close or get rid of him.