Difficulty settling 2yr old into nursery - help!

[lottytheladybird]

We've decided to let our DS1 go to nursery, as DS2 is just 6 months old and it'd be quite nice to have some time with just the baby. So far, we thought we'd let DS1 go just one morning a week. He's been 4 times so far and it feels like we're going backwards.

Here's how it's gone so far:
Week 1 - Happy to explore the room and played outside without me for 20 minutes.
Week 2 - Wasn't happy, but not unhappy either. Spent just over an hour without me.
Week 3 - DS1 wasn't upset when I said 'bye', but got upset later and continued to be upset until I came back. I was away for about an hour.
Week 4 - DS1 wouldn't let me go at all. Every time I left, he got upset and I had to come back.

We're thinking about letting DS1 go one more morning a week just so that it won't be so long between sessions.

I'd be really interested in hearing your experience of settling your DC into nursery. Also, do you have any tips on settling a child into nursery with minimal tears?

(DS1 may be on the autistic spectrum and has delayed speech, so that doesn't help...)