I'm child free, by choice. I chose NOT to have constantly screaming children as part of my life. Therefore, I shouldn't have my sleep or my working day constantly interrupted by earth shattering screeching. My life choices don't impact upon anyone else's life, why should someone else's affect mine? That's not a neighbourly or considerate way to live in a built-up suburban neighbourhood.
Whilst I don't fully know or understand your or your neighbours particular situation, I can tell you, that from my perspective, I can understand his frustrations. I almost stomped 'round to my neighbours in a rage, and full of tears, at 4am the other night.
So... my next door neighbours leaves their child to cry and scream, full pelt, for hours on end, day and night. I was in tears the other night when it got to 4am and I hadn't managed more than ten minutes of sleep and had to be up to work in two hours. It's SO draining. Baby crying is the soundtrack of my life now. I'm actually getting depressed coz of it. I'm either crying myself, or raging. It's awful.
I think the child's cot is up against the adjoining wall, literally behind my bed (my home office is also in my bedroom, so there's literally no escape). I can hear the parents chatting and laughing sometimes, but rarely hear them soothing the child when it's upset.
I've lived in this flat for decades and had many different neighbours over the years. I've heard babies cry sometimes, but when it's life alteringly frequent, like this is, it's soul draining. It's all well and good for mums to say, "What does he expect you to do?" Well what do you expect us to do? You love your child so the crying won't feel like it does to others. It's such a hideous sound, that's why it's used as a torture method on prisoners of war.
Fortunately, the law is on the side of the neighbour, and should he take his complaint to the council, they will investigate. They'll look into the frequency and noise levels, etc.
Of course, occasional, 'normal' crying baby noise, though still horrid, is sort of an expected part of being adjoined, but constant, screaming is another thing altogether.
I'm going to pop a note in to my neighbour explaining how it's all affecting my life and that if things don't improve over the next 28 days, I'll involve the council. And, I think that's fair enough.