A.S.
Choose the religion you want based upon your beliefs and and the faith that develops in your heart as you learn about it and believe it's truthfulness. Pray for guidance. Feelings of the spirit can testify to you the truthfulness of a religion or not, but only after you understand it's history, it's future, the belief system.
Joining a religion for convenience, to get married... can only cause contention later on down the road if clashes arise on certain subjects, upbringing of the children, deep rooted differences on what your beliefs are...
Also, he may be he same great guy, but people change over the years and short visits may not reveal those unsightly changes. Get to know him a lot better and go to where you are going for you and your child's own benefit, not his.
At this point, you are considering major life changes, moving, joining a religion and getting married, based upon a former brief relationship with a man who has only implied he would reunite with you.
Take the relationship slowly as it comes, and despite how exciting it may feel right now, realize that once you are there, things could quickly change with him. (not saying that things will of course, just be prepared for that).