Very long middle night wake ups

Hi all

After some advise about little ones sleep. Currently 21 months and has the most random sleeps . Generally going to sleep is ok, takes anywhere from 10 mins to 30 mins and we walk out no issues .

The next part is the problem. Sometimes he sleeps through til morning no problems, sometimes he wakes up once early evening for 20 mins and then sometimes he wakes up middle of the night for hours.

Currently this week mon-wed slept through , Thurs was up at 9 for 30 mins and then 12-4 . Fri slept through and tonight he woke up at 3am and it's 5pm and still sat here.

Have a nearly 6 year old who sleeps fine. Used the same routine and he just slept through from about 14 months

Naps from around 1.30 to 3 every day but can alter slightly. We try and limit it to 1 and half hours but if he's been up hours in the night it can be slightly more and slightly earlier.

Bedtime starts at 6.30 or 6.15 (bath night every other day) with him in his bedroom at 7pm. 2-3 books and a 3 minute cuddle afterwards. Dark room with one of the egg temp sensors on low , very quiet music playing (normally some sort of classical) roughly around 18c and wears and 2.5 tog sleeping bag

Nap wise we have no issues getting him to sleep normally takes 10 mins

We are very consistent with this and have been for well over a year.

If he wakes up past 12 he is always up for hours, we pick him up for 2 mins to stop the crying and sit next to him until he falls asleep. This normally takes a while, and then try to leave and he will instantly wake up stand up and cry. This can happen several times. He will literally be snoring and fast asleep but will dart up when the door closes

I have no idea what to do. I want to sleep train him by leaving the room and coming back every 5 mins etc.. but read that isn't the best thing now days ?

Me and wife normally take turns with wake up or swop half way through. He is a mommies boy but doesn't seem to make a difference who is with him. Wife has been poorly all week so I've done them all every night and had enough

Any advice is appreciated.