YES on getting a buddy!!! Some people/vets say it's not necessary, but general consensus on this forum is that they NEED a friend! You are his human buddy, and while that is a fabulous thing to be, most members here believe that you can't replace his need for a buddy of his species. If he is lonely for a glider companion, it could certainly be the cause for his depressed-like state, though it wouldn't explain its sudden onset after the nail issue.
I have read that if the nails get clipped too short, that they can have a hard time latching on to things, and with balance etc. I don't know if this is true, (maybe someone else can verify?) but if it is, maybe your glider is having issues with this and is scared of falling??? Or maybe you're right and he did have a traumatic experience. Either way I hope things resolve and that you can get back to happy parenting. Will be thinking of you and your sweet baby.