Gangle, sympathies.
My dd's sleep took a dive around 4-5 months. I think it is because her sleep has always been crap but she was still in the sleepy baby fug. Once she shook that off, it was no holds barred.
Yes, around this time, I would nurse dd to sleep and she would wake the minute I try to put her down. My solution was to move her to a bed (have bed guards, pillows on the floor etc if you are concerned about her falling off). I would nurse her off the sleep lying down and then try to sneak off. This way there is no lowering of the body which wakes them up. In my dd's case, she would not let me sneak off. So I co-slept with her. But at least she did not wake, not at least for another hour.
As for not sleeping in a pram, I found that by this time, because she is so much more alert, she will not fall asleep in a buggy unless it was actually her nap time. Any other time, she would not be able to, get bored in the buggy and start to fuss. So I would time walks around naptimes.
My dd would wake when the buggy stopped moving. So I did a lot of walking in those days.
Do you think he could be teething? A bit early but not unheard of. He could be uncomfortable even if there are no obvious gum signs. Pre-teething, where the teeth are moving under the gums. One sign is when he nurses, his latch changes either fluttery or bitey. Calpol, calgel, teething powders.