HELP! How do I get my Toddler to sleep more than 4 hours a night!

I am desperate for any ideas/ suggestions on getting my 2 year old to sleep longer than 4 hours at night.
Our routine has been the same for about 12 months or so
5.30pm - Dinner
6.30pm - Bath/ Shower
7.00pm - Read a book, sing a few songs, light goes off, i say goodnight, he has his teddy and a milk bottle and i leave the room

From here he will either:
Get up, stand at his gate and shout for attention/ throw things out the door or cry. I tell him its bedtime, back to bed. I go in if he is too upset, lay him down and tell him good night.
Other nights he will fall off to sleep straight away.

For the past four weeks he has been waking at 12.30am/12.40am every night. He will then either stay awake for the rest of the night crying while i am up and down trying to lay him back down, other times he will wake back up every half hour or hour. He usually wakes up quite upset. We have always left our laundry light on which lets minimal light in his room.

He is at daycare full time and is very active. He eats very well and has a 1 hour nap during the day.

He's only getting 4-5 hours broken sleep a night. I'm concerned about what effect the lack of sleep will have on his development, health etc

What are some reasons he could be waking and how can i get him to sleep longer? Do i need to change our bed routine?

I am exhausted! I'm getting about 3 hours broken sleep each night, while working full time and trying to study at night.

Thank you!!