Adolescence complicates everything. There is a book called Divas and Doorslammers, by Charlie Taylor, in which he explains how the adolescent brain is rewiring during those years - so there is real change going on, and this can lead to them losing some abilities during that time - things like impulse/temper control, empathy, the ability to see that they aren't the centre of the universe.
He describes it as almost a form of temporary brain damage - but it is temporary, they should get these abilities back once the rewiring is done and things settle down in their brains again.
Whilst this doesn't help with the issue of depression or being bullied, it may help explain why he gets rude with you when you try to talk to him. Ds3, in particular, went through the rude and thoughtless stage in adolescence, and knowing about this brain issue helped me to cope with it, because I knew it wasn't the real him, but the rewiring him that was calling me names.
You also asked what the parents of bullies' victims could have done. In my case, I wish my mum had taken it seriously, and had allowed me to take it seriously too. I wish she had realised how damaging the name calling and exclusionary behaviour was.
I wish she had told me that she'd go with me to talk to the teacher - I was in a tiny primary school (10 in the Infant classroom and 18 in the Juniors, and the Junior teacher was also the Head). I am 100% sure the Head would have dealt with it firmly and decisively - but as I said in my earlier post, mum was so dismissive of me that I had no reason to think that any other adult would care more about what was happening to me - or that, if I did go to the Head, and there was a backlash, that mum would support me.
I also wish that mum had checked up on me - whatever we had decided to do about the bullying, it would have been good if she had asked me if things had improved, and had made me feel I had her full attention and that she cared. I wish she'd noticed how withdrawn and lonely I was getting.
I don't see how you can fail to be aware that your child is depressed. Ds3 went through a hard time during his second year at university (and eventually moved to a different one, nearer all his mates), and I could tell, just from talking to him on the phone, that he was depressed, so I offered him all my support and was there for him. And when ds2 was being bullied at primary school, I knew, as soon as I saw him in school (whilst I was in reading with children from ds3's class, and saw ds2 and his class walking through the hall), that there was something wrong. He was a pale, withdrawn shadow of himself. I hadn't seen it before, because it was masked by his relief at getting out of school, and being with his friend, who came around to ours a lot.
But as soon as I saw him in school, I knew there was a problem, and asked him about it that afternoon - and the next day I sent a letter in to the school, detailing the problems and the bullying he was receiving, and made sure it was dealt with.
I can't see how my parents can have been unaware of my mental state, even if they weren't aware of exactly how bad it was - but I suspect my mum did and said nothing because she didn't want the hassle of having to deal with the school and any fallout. Maybe that is wrong and I am being unfair - but that is how it feels - even now.