Night waking 1 year old

Hi

My son turned 1 in August.
Over the last week, every night between 4am/5am, he wakes screaming.
I will normally hear a couple of crys and leave him for a while to see if he settles himself, but he doesnt, he just screams, and crys and crys.
I will normally go in and put the dummy in his mouth and place my hand on his chest, he will normally lie there for a little bit, but when I walk away he will start screaming again when he realises I'm not there.
I have tried feeding him up at night, thinking he is hungry, tried giving him a bottle, which he will settle for an hour, but then just wakes up screaming again. The only way he will settle down again is when I hold him really tight next to me in bed and he will then normally settle down again.
This is really unusual behaviour for him as he has good sleeps and will normally settle himself to take his naps.
Its almost as if he is not wanting me to leave him.
He has only been doing this since he had a hernia operation a couple of weeks ago. Is he having nightmares.
It seems strange that it is the same time every night.
Please help - I want my sleeping boy back. He used to sleep from 7pm to 6.30am.

we have a 2 1/2 year old who is waking once each nite - falling out of bed we think...as i picked her up off floor the other nite...ugh...

if we can get her back to bed quicksmart we do...worst case we cuddle up in dark and one of us takes her back to bed when she is asleep again...usually within mins..

we have been leving a nitelight on more recently...with age comes awareness (of dark etc) and i bought her one of those dora torch lights for beside the bed that she can use if need be....