Hi Terry,
Space Mountain (SM) is one of my all time favorite rides! Once upon a time, the ride vehicles in SM were designed where 2 people sat together in 1 seat. The larger of the two sat down 1st, was seat belted in & then the smallest person sat between his/her legs & another seat belt was put on. That has changed - now 3 people sit in 1 of the 2 connected ride vehicles so there is 6 total. Everyone sits in individual seats behind each other & only 1 person is allowed in each seat. You could put your son in front of you but that is as close as you'll get.
There is a minimum height requirement of 44" to ride SM. Keep in mind that just because a child is tall enough to ride a ride, doesn't mean that it is appropriate at the first age they meet the height requirements. Height requirements are set keeping safety purposes in mind.
As a warm-up, ride Big Thunder Mountain Railroad with him. If he doesn't like that, I don't think he'd enjoy SM. Also take him on the Tomorrowland PeopleMover which goes through SM so he can see just how really dark it will be during the ride. If he still thinks he wants to go, explain to him that SM will be super dark, you can't sit with him, it has many twists, turns & small hills but no huge hills or loops. If you think he's ready go for it. If he gets in & sees the ride vehicles & wants to back out, there's always an escape hatch for those space travelers that want to abort the mission.
I've had parents tell me that they've taken their 5 year olds on SM & they loved it - others were terrified. I don't think my 6 year old would be ready but you know your son better than anyone & he may be a thrill seeker.
Wishing you many magical memories :-) Allison