Most abscesses don't require the eye to be removed, the swelling typically goes down after 3 or 4 days of antibiotics.... however sometimes the eye does need removed.
A friend of mine just went through this with one of hers she will be able to help you way more than I can and guide your through the cleaning and after care she has had to do...you can message her on facebook n just tell her Carla sent you here's her link
https://www.facebook.com/profile.php?id=100004843098345
Weight loss and sleeping more is normal when they are dealing with an infection I wouldn't worry to much about that.
Is he separated from his cagemate?
how is he self mutilating?
Putting him down is a very hard and personal and most unselfish decision your going to have to make yourself.
There is no one here that knows him like you do and there is no one here that is there to see how the procedure goes for him but you.
All I CAN say about that is gliders have had it done before and recovered easily and went on to live and thrive with a good quality of life. However, he is getting up there in age which increases the risk of him not waking up from anesthesia, the older they are the more risky it is to put them under, that would be my biggest concern if he were mine. I hate that you have to make this call, it is hard believe me I know first hand but I would talk to Denise before you make any final decisions, she not only has experience with eye removals but also older gliders and self mutilators, she has a wealth of knowledge and experience you couldn't be in better hands with her.