we had a similar experience for ds1, he went to a childminder when I went back to work when he was 12 months, he did 4 days for 6 months then 2 days to 'retain place' after ds2 was born, childminder gave up minding so he started day nursery when I went back to work (started very quickly, minder gave notice over xmas shutdown period, grr, no chance to settle ds1 or 2 in properly) for 3 days a week when 2.5yrs, went there for 7 months which overlapped with ds3 being born and me stopping work again, he then started preschool (5 mornings/week) and there now, due to start at school in Sept.
The move to the nursery went really really badly, it took ds about 4 months to go without crying (and I cried) we toughed it out as I had to work and I knew I'd be finished and he could stop, by the time I'd left on mat leave he was really happy there and stopped asking about childminder.
On his first day at preschool I got dh to take day of work to have younger 2 so I could focus on him, we'd had one visit in summer term before, I'd bought him books on starting school and we'd walked up to school a couple of times so he could see the play ground, gate he'd go through etc. Basically I was totally prepared to take him straight home if he cried, I'd decided he'd had a stressful year and he didn't have to stay if he was upset, told him I wouldn't leave him if he didn't want me too and he didn't even give me a backward glance! has loved it from day one. I think for us the key was preparing him, I felt happy I'd done everything I could to make sure he knew what was going on and that made him ok with it - you have to be happy with the decision, I'm a firm believer in happy mum happy baby.
Plus I think because I'm on maternity leave I had luxury of being able to spend time settling ds1, if he's having a wobbly day I don't rush him, when I was working I couldn't stay to comfort him I had to be at my desk, ds1 has never been upset about preschool and I think its because of that vibe (and he's young enough not to take advantage of it!)
sorry for the long post but hope that helped - not really any practical advice but I can emphathise!