I need some insights

I'm in a very serious relationship and a very close friend (also an ex) asked me to be a surrogate mother for him. He lost his son 2 years ago and wants another child but doesn't want a relationship or sex. He just needs someone to carry the baby. I wouldn't be doing anything sexual or even touching him. I am not sure how to bring this up to my fiancé. My ex is a friend still and I feel honored he wants me to do it and I would love to but it's kinda weird ya know. Any input? Please no negativity.