Hi XXXXXXXX~
It's not unusual for an older dog, or one who isn't feeling well, to want to be by themselves and not be touched.
Part of it could be that he's sore because of the spinal issues and the constipation, he could also just be feeling sickly because of the allergies.
I know that when I'm not feeling well, I don't want to be bothered in the least. I want to curl up on the end of my couch with my favorite blanket and the remote and just have everyone leave me alone until I feel better. My sister, on the other hand, wants to be cuddled and pampered when she's feeling sick...dogs are very much the same way.
Hopefully once your vet gets him all figured out and on some medications to help with the allergies, he'll start to feel better and will want to share your bed again.
I hope this helps.
Certified Veterinary Technician
Associates Degree in Veterinary Technology. State and Nationally certified.