Hmmm...could you start bringing familiar things to the house (literally one at a time, and with him) to stay there?
Not his things at first - that would probably be too traumatic (and he might not understand?). But things he would recognize as belonging to home, especially when he would see them again at subsequent visits right where they had been left; Turning the new house into a familiar place and bridging the gap between old home and new home, hopefully.
I can't really explain why I suggest this more than I already have except to say it would have helped me when I had no language yet, and even after I had language as a child. (It would probably help me now, actually)
I still have immense difficulty with change and with challening environments...
Another thought is:
Does he have sensory sensitivities? If he does, or if it is unknown but he might, maybe try to do a sort of sensory inventory of the new house to see if for example:
Are there ambient noises inside or outside that might be upsetting him? (if he has hyperacusis like I do, and which was much worse as a child, noises you cannot even hear at all unless outside, he might be able to hear them inside and they might bother him a lot). [Normal inside the house sounds that can also be very awful: dripping tap; furnace; radiator buzzing or clicking rapidly; refrigerator or freezer or other-always-on-appliance buzzing; lights buzzing; electrical outlets buzzing...] Simple ear defenders can help if he tolerates wearing headphones (and doesn't already).
Are there new flooring or paint smells, perfumes, food smell, that are unfamiliar or that he might find aversive? Something that is barely noticeable to you could be overpowering and intolerable to an autistic child with severe sensory sensitivities. Even my father who has sensory sensitivites himself has always been amazed by the acuity of my sense of smell and hearing.
Sometimes glare or colors or lighting (too bright, too dark[; "warmth" or "coolness" of lightbulbs; mind-shattering, literal-crazy-making flickering (fluorescents are worst for this - they make my brain stop working and give me migraine headaches)] ) can be issues...
Another thought is to try things like painting/decorating walls at the new house the same colors/wallpapers/etc as you had in your old house for the same or similar rooms/areas. (Ideally exacly the same -- like take a photo a miniscule chip of paint and try to get it color-matched.) Or maybe trying to create similarity in flooring, if possible. (Like if new house has hard flooring and old house has carpet a maybe even trying to find big area rugs similar to the old carpet would help?)
If he is only comfortable at home, really try to make the new house as much like the old one in sensory terms as you can...that could address both sensory sensitivity reactions and issues with change at the same time.
Good luck to you and your son, to your family!
