How many settling in sessions has he had?
My nursery has a very flexible approach to starting. DS2 was about 18mo when he started, he had already been at another nursery but we still had 2 sessions, one with me there and one with me lurking round the corner.
DS2 has just started at 6m, we had 1 hour long session with me there and then 2 short half days (about 3 hours, one morning and one afternoon) with me "on call". If there had been any problems they were clear that we could have more settling in sessions as well with me there for all or some of them.
Have you talked to his keyworker about ways of helping him feel more at home? My nursery asked for loads of info on this eg favourite songs / games / story books.
I think your DS is at a difficult age to settle really so some more short sessions might be helpful? Also, how accustomed is he to being away from you? Even you having short solo trips out to the pub shops etc can be used to teach him that sometimes you go but you always come back.
It's hard to strike a balance at drop-off sometimes between just sprinting out and upsetting them further or staying longer and them working themselves up into a frenzy. Personally I do the same routine whatever DS1's mood (9 out I 10 times he is fine, 1 out of 10 he gets a little bit upset) so into the room, cuddle, talk to
Keyworker, find an activity / toy, few more cuddles then off. If he cries (very very rare now but more common at your Ds's age) I always wait in the building (out of sight) until he has stopped - has never taken more than a minute!
My nursery is not just one room with a weekly walk either! The reason I prefer it is that they do loads of stuff that I / a childminder couldn't do eg loads of age-specific messy play / incubating chicken eggs / lots of pets / free access to the garden / massive sandpit & loads of different play equipment etc. He's also with a bigger group of his peers not a couple older than him plus a baby. I think that's useful.