Hi, welcome to BW
Can you write out his day - naps, etc.?
Does he nap in his cot?
Any medical issues? This sounds like there's some pain or something involved, even a child who has lots of props will generally sleep for a period of time and a very OT child (which I'm sure your LO is!) will crash at some point and sleep.
Well today is a early sleep day due to being up half the night but usually it's...
7am - wake up and bottle
7-10/12 - play time, walk
10-12 - sometime between these two times he will nap for an hour (usually sofa or pram as out and about and wont settle in cot)
12.30 - lunch (usually sandwich)
1-3 - play/walk to school
4pm - usually another hour nap
5pm - Dinner time
7 - bath time, books, lights down
7.30 - Milk 7oz
8pm - 11pm - trying to settle for sleep. He has fallen asleep twice in his cot at bedtime but usually awake within an hour.
12.30am - screaming, trying to settle without milk but usually have to give in. Put him back in cot but up and down to crying until I give in and get up.
Has trouble poohing sometimes, he is on lactulose and movicol on and off when he struggles. He seems to be fine during the day it's just once he falls asleep for night time he starts. I haven't done any sleep training yet as he gets so upset and I worry about my 5 year old sleeping. He is generally a really happy baby especially during the day, we thought now he is getting himself mobile that he would be so tired he would want to sleep...I was wrong.