DD1 slept like an angel. DD2 was always a shocker at night - with no explanation.
As a newborn she would cry for 3 hrs after a night feed, finally go to sleep, and be due for another feed sad
By 3 months she wasn't crying for as long.
Have come to the conclusion its a personality thing - was always full, burped, changed etc - didnt help if we held her, rocked, patted etc, and always had a dummy and favourite toy. She has always been very happy during the day awake or asleep.
At 12mths we added a dull night light and quiet 'bedtime'music to the routine. she was still waking about 3 hrs after she went down and crying for up to 3hrs. One night I had had enough and told her to "be quiet, everyone is trying to sleep, Mummys going to bed, GOOD NIGHT!!"
She cried, but I left her - peeking in occasionally to check she was ok. It was hard to listen to, but seems to have worked. From then on she still woke but settled much quicker and easier.
At 22mths she still wakes most nights at about 10pm, but is asleep again within 15mins. She just isnt a good sleeper.
Would reccommend the dull night light (we use rechargeable ones from Kmart\woolies) as my baby always jumped up when her door opened and the light crept in and cried when she saw the door shut - lets you peek in without being as obvious.
Choose a weekend to try this so you have some back-up and perhaps send DS for a sleepover if possible so you only have one to deal with.