Hi everyone, I'm looking for some advice please.
Our DS (I assume that's darling son?) is now 10 months old and has been in his own nursery room since about 6m. He's always slept well, usually waking for a single bottle around 3-4am, and getting through a good 10-11hr night uneventfully. Until about a month ago. He suddenly developed what we think is separation anxiety - perfectly typical for a 10m old it seems - evidenced by his immediate crying if he suddenly finds himself alone, or catches us just nipping out of the room (toilet, tea etc.). This is understandable, from what I've read they're becoming self-aware and recognise their dependency on us parents as care-givers and get scared when alone. Unfortunately this has led to a complete change in his sleeping behaviour.
He now wakes many times in the night, crying instantly when he wakes, and he takes a lot of settling back down, only to get anywhere from 2-3 hrs to perhaps just 5 mins, before he wakes up again. Some nights we simply have to bring him into our bed. This doesn't reduce his crying/wake periods, but it's easier to resettle him if he's right next to you and not in another room. A second reason we do this is we have another elder son who would get disturbed if he was subjected to crying baby sounds all night long.
Reading about SA I see that it can last years, which worries the heck out of us because we're at the end of our tether already. My wife and I are sleeping in separate rooms and taking turns each night to be 'on duty', however we're both tuned to the crying so even when it's my turn my wife still gets disturbed herself at each wake/cry event. So we're not sleeping well at all ourselves, and that's affecting our welfare/work/family life.
Today is Christmas Day (Merry Christmas everyone 
!) and DS fell asleep tonight at 9pm after his last bottle. We transferred him to cot at 9:15pm and enjoyed about an hour before retiring to bed(s) ourselves. 11:30pm DS started crying. I've been into his room twice already and it's only just gone midnight. I'm expecting another long broken night 
.
We have no idea how to ease DS's sleeping such that he doesn't wake so terrified that he feels he has to cry out for us (his crying sounds like a major sobbing/screaming, like he's been alone for days!) despite it only taking a couple of seconds to get to him each time. We've had advice from some to just leave him to cry himself back to sleep, but I've seen what that does to a child, they become distrustful and distant because they believe they're not loved because nobody comes to them when they're afraid and crying out for help. According to Separation Anxiety discussions he's literally terrified and in need of comfort, a safe place, and right now that's us, his parents. So to leave him scared overnight to sleep through sounds from exhaustion seems like cruel torture. We won't entertain that idea.
We've tried leaving an object, like clothing, that smells of his mum, but that doesn't work. We've tried fewer layers of clothing, more clothing, sleeping bags, baby duvets/blankets, teddy bears, different lying positions, different lighting (off, nightlight), nursery door closed, nursery door ajar, nursery door wide open. We've tried over-feeding him before bed, refusing a feed in the night (that one was a bad idea), keeping him up until as late as we think he can bear it before letting him fall asleep at night. But nothing changes this behaviour.
If it matters:
He's on solids/baby food/our food, three times a day with bottles (SMA) in between (usually 4-5 bottles a day).
He used to wake for one feed in the night, now it's two (1st around 1:30am, the next around 5am), and he drinks about 120ml each time (he won't take any more, despite the fact we know he can empty a 180ml bottle 
).
On an afternoon, anywhere around 19:30 to 22:30 depending on his mood, he'll want to fall asleep, so we give him his last bottle and cradle him to sleep. This happens in our arms in the lounge, and after half an hour asleep on us we transfer him to his cot and sneak out of the room.
Only twice have I ever managed to put him into his cot awake and leave him to fall asleep alone, and that was months ago when he was much more settled (If I tried that now he'd scream the wallpaper off the walls). If he ever woke and noticed us leaving him in the nursery alone he'd be up like a rocket and crying instantly.
As a baby he's unusual in that he's only slightly above average weight, has so far reached typical baby milestones on target, has an interest in everything he sees and can crawl to, doesn't like being reprimanded, loves to touch everything that's not a baby toy, finds the dog hilarious, loves being outside etc. etc. but he just will not any longer sleep through the night.
I see on here some are talking about "sleep training" - I have no idea what this is, but maybe something I need to look into? Also, some talk about "co-sleeping", which I presume means DS sleeping in bed with us all night? Is that usual? Does that not mean you get zero sleep yourself!?
This is an enormous post, I thank you for reading it all, and I thank you all in advance for your thoughts and suggestions.
Signed: One very tired and nearly-broken father.