Hi there
Can you run some white noise (mp3 track, radio tuned to static, fan turned to the wall) in the baby's room so that DS is less likely to wake her in the middle of the night? Do you think he is overtired? Does he still nap (when/how long)?
When he wakes at night, what happens? How do you settle him back to sleep? Is he still in a cot or in a big boy bed now?