How can we get him to self-settle?

[Calluna]

I know I dont really have much experience in this but thought I'd share just in case it helps

I used to babysit a little boy about your DS's age and what worked really well with him was:
do the night time routine,
read 2 short books
say goodnight,
tell him I'd be back in 2 mins and leave.
Come back in 2 mins and put my head around the door to tell him I'd be back in 5 mins, 7 mins, 10mins. If he still wasn't asleep after 10mins, keep checking in 10 minutely (is that a word? Lol).

Only once did I have to check him again after the 10 minute one and most times he was asleep before the 5 min check

HTH