Reading this thread has helped me a lot. Seeing how so many people have reacted to a parent treating a child like this has really put things in perspective for me.
My mum did something similar to me when my mum and dad split up (I was 3)...from then on increasingly leaned on me as if I was a partner, emotionally, practically, financially.
I don't blame her as such- she was desperate, lonely, depressed...her father died at the same time her marriage split up. I think she probably had undiagnosed Aspergers or similar looking back. She really struggled. There were decisions in which she very much put me first, and which I am very thankful she did that.
She did also make attempts to move our dynamic on from the dynamic evident between her and her mother (very controlling), with partial success. so it wasn't all one way traffic, a lot of it was needs must in a hostile landscape.
It has shaped a lot of my life, I made a lot of sacrifices, witting and unwitting. For a long time I felt I failed in that I wasn't able to fully support her to be "recover". This thread is helping to lessen that sense of failure. I did my best in a mess that wasn't of my own creation, in which a lot of the adults involved had totally unrealistic and often selfish expectations of me (I include my father and my extended family in that equation as well as my mother).
To the OP, I would say, when she was dying, my mum did apologise to me for having emotionally neglected me as a child. She both thanked me and apologised to me for the amount of support I had to give her. It did mean a lot to me, and has helped me move on since her death. It would have helped more if it had come sooner, if it had happened earlier in my life.
So apologise, express pride in him and gratitude for him. And in word and action set him free.