- 3/4 Celsius during the night. This is about 26 Fahrenheit, just a little below freezing.
I've had chicks 5-1/2 weeks old go through the night in similar temperatures with no heat, so he is old enough. When I did it they had great ventilation up high and really good breeze protection down low where they were. I also had several so they could huddle if they needed to but I don't see that as all that necessary. I've seen grown chickens sleep by themselves in trees at -20 C. What keeps them warm is their down coat, not sharing body heat with others. I personally would not have a problem having him out there by himself because of your temperatures as long as the ventilation and breeze protection was good.
The flock has a alpha male and he is really aggressive so I'm not too keen on grouping with this youngster.
Not sure what you mean by really aggressive. Odds are any issues would not be with most adult males, I consider the hens more dangerous to a young chick. At eight weeks he's probably still young enough that the rooster will see him as a chick and not rival, but you never know for sure. And he is going to keep getting older, less of a chick and more of a cockerel. That can develop into a conflict with the old boy. Hens can be real bullies to any chickens weaker than them, mature roosters usually not so much unless it is a rival.
Mary is right, I think your real issue is integrating a single chick into the flock. Before too long he will hit puberty, At that time he can run into problems with the hens and maybe that old rooster. I don't know what your facilities look like so it's hard to give any specific suggestions. So some generic ideas. House him across wire for a while, then let him loose to range with the flock. Separated feeding and watering stations can help. Give them as much room as you can. Clutter in the coop and run can help. By clutter I mean things he can hide under, behind, or over. As much as you can let them work things out at their own pace. Do not try to force them into close quarters, all that does is raise the risk of a disaster. They will not fight to a solution. Try to be as patient as you can but get started. In spite of all we say sometimes these things can be really easy. But they can also be pretty rough. Try to be around and have a place ready to isolate him if you need to.