It’s stress and depression from being alone. I’d say get him a female and get him fixed if you don’t want babies. Males tend to be aggressive if they aren’t fixed. You cannot fix a female it is impossible so don’t believe anyone who tells you otherwise. Their urinary tract runs through the reproductive. We have rescued 4 with the same balding issues. After a month of intros then a few weeks with the colony hair growth began to start. Introductions can be very difficult and you can’t expect it to go perfect everytime. We have one male that won’t bond to a single glider and there’s 9 others. Make sure if you get another glider it is the same size as him, if you get him fixed before you get another then gender doesn’t matter. For the start you will need 2 cages. You have to scent train them for 3-6 weeks. After 3 weeks of switching toys and beds you can show them to eachother while they are sleepy and see how they react. A hissing then click noise is a good sign, it sounds scary but it’s actually very good. If that noise occurs do not keep them in separate cages. If they fight, separate them. From the intros I’ve done females tend to bond the quickest to other gliders but they are slower when it comes to humans. Every glider is different so keep that in mind as well. Plus gliders are colony animals, they live longer and so much better mentally with a friend or two. Since he is 3 years I’d look into rescues as well, all of ours that had been balding “stress grooming” have bonded quickly and helped eachother a lot to get better.