sleep problems

Hi

One of my twins, now 13 months, has been diagnosed with severe CP. I can deal with hte muscle spasms and involuntary movements, but his inability to sleep is driving me nuts. We have got to the stage where I can put him to bed awake during the day for a nap or for night's sleep, and he get himself to sleep. However, when he wakes up during the night, it takes a minimum an hour to get him back down. There are nights when he is up 3-4h in one go, and nights when we are up 6 times or more, trying to settle him.
So looking for your wisdom re: sleep. What have you found helpful in getting your LO to sleep?

Thanks!