8 month old - seperation anxiety?

[LONC2012]

Hi all,
My DS is 8 months old, until around 6 months he slept really well and it has all gone downhill from there.
Up until a few weeks ago he was waking once or twice but now is getting to every hour.
Sometimes he goes to bed asleep and other times awake depending on his naps in the day. The past few nights if I have put him down awake he has screamed as soon as I leave the room.
I have then been sitting with him till he is fully asleep and then putting him back, I have tried to calm him down stood by his cot but he isn't having any of it. As soon as he is up out of the cot he stops crying.
Last night this was every hour he would wake crying his eyes out.
I am absolutely exhausted. I have another little boy too who is 4 so when baby wakes I try to settle him as soon as I can so I don't disturb his brother.
I'm literally stuck at what to do, I want to nip it in the bud as soon as possible before he gets too used to falling asleep in my arms.
Has anyone else been through this and got any tips?
Through the day he usually cries if I walk away, I try talking to him the whole time I'm gone which sometimes helps but not often. We frequently play peekaboo, I just don't know where to go from here. I'm so bloody tired!