He's not likely to be gay to begin with, so the person he will seek out as a "soul mate" will be female.

Even in the unlikely event that he is gay, the chances of him choosing to remain with you for ever rather than dating other men as he gets older are also quite small.

The problem isn't that he isn't a person. The problem is that he is! As a person, he has his own interests, needs, and desires. What are the chances that his needs, interests, and lifelong desires involve moving in with an older man and spending the rest of his life in a celibate relationship with him? As a person with his own hopes and dreams, he is far more likely to desire a spouse and children, a family of his own.

Yes, it's possible you may wind up being best friends. But unless your plan is some psychotic murder-suicide scheme, it's not going to solve your "don't want to die alone" problem. You will have to find another to solve that.