I never told my son that my fiancรยฉe was my boyfriend or whatever, I just introduced him by his name and he just started hanging out with us little by little at a time, and now my son is crazy about him and calls him Daddy and everything. I don't think you should over complicate things for now, and limit kissing and stuff at first because kids are territorial with their parents. Just have them meet and hang out at first, (we went to Coney Island) go out to eat, etc then he will feel more like your boyfriend is part of his family and less like someone taking away his daddy.
As far as being gay, I think you should just wait for your son to ask you about it and then answer his questions honestly in a simple way. Most of time we as adult over explain things to kids and often when they ask you and you dig deeper you will see how simple of an answer they really want! As he gets older you can explain it to him better, but I don't think he will reactive in a negative way unless that is how you come across about telling him.