in fallout new vegas how do you get rex?

Answer 1

all You need to know about him is here http://fallout.wikia.com/wiki/Rex_(Fallout:_New_Vegas)

Answer 2

you have to help the king out and do missions for him and then eventually he will give you the mission to get rex a new brain. if you successfully complete this mission the king will let you have him.

Answer 3

Do enough misions for the king and he will be given to you.