it's called ..... life happens.
Just begin again with bonding, bonding bag time, tent time. Get to know him again and let him get to know you again. New toys will help, foraging and stimulating toys. Easter eggs with treats in them are great for foraging and it's nearly easter so they will be out in droves. Braided rexlace or fleece so he can un do it. Reset toys pompom pits etc... I'm sure he will love anything new and stimulating.
Gliders over groom for many reasons it may not be depression at all. I had one that would over groom if I changed his cage around to often, it stressed him out but for other gliders they like it changed around often so if you haven't done that he may just be bored. Some will over groom for other environmental changes, lights, noises, new pets etc... it's a broad category and can be hard to narrow down the actual cause.
Now most people would start out with "get him a cage mate" which I agree would also potentially help. However, there is no sense getting in a panic over a new glider until you get back on track with the current one. Unless you are ready to tackle both at the same time but that will require tending to two cages and two gliders bonding with you.
It's a process but he would have company 24/7 when life gets busy for you again and it probably will cause life does that. It's still easier said then done especially if he is intact, then it will also require a neuter before any intros.
It is an option, But that is up for you to decide on if your in a position to even do that.