I suspect that the answer to this is because he's four months old...
DS has had a bedtime routine since six weeks and it works brilliantly for him. He sleeps happily in his cot, with a couple of feeds and usually cosleeping from 5amish when he becomes a bit unsettled. We were really lucky to be able to put a loose structure in that helps him so early. Daytime naps have been tricky the past week so have tried an adapted version of bedtime for naptimes today - out like a light. I can't then put him down but that's another issue.
He's taken to crying while he actually falls asleep at bedtime. We do clean nappy/pyjamas, say night night to everyone then bottle and bed. He'll fall asleep either on his bottle or having a cuddle, then wake up as he's being put down and go to sleep in his cot. I leave the room while he does this and watch on the monitor as I seem to distract him otherwise.
All fine and still okay apart from the initial dozing off - he can't seem to manage this without getting really upset and crying first. This is new. Why?! And why just at bedtime - he's fine during the night. I've tried putting him straight in his cot and settling there but without any luck, I think he's not quite there yet, but he has enjoyed playing with his blanket during the attempts.