My daughter had never been to daycare or preschool and did fine in all-day kindergarten. I think all day kindy is actually easier because there is more time for specials, snacks, recess, etc. Academically, she was pretty far ahead but the novelty of being with kids all day, routine, art, music, recess, etc kept her engaged.
Kindy teachers expect a lot of kids to have not been in preschool. I mean, it's kindergarten, if it's not built to be the first school experience for at least some of the kids, they did it wrong.
I was a bit concerned about this as my 4yr old was on the younger side starting jk and had only ever been in daycare part time (then I changed my schedule to straight nights so that my boys didn't have daycare at all. I shouldn't have worried, he has loved it. it was harder for me to recognize that my baby was going to be in school full day when his older brother had the 1/2 day (2.5hrs) option.
I did do two half day camps before school started for both to get them used to the routine idea. with a great sigh of relief, am glad to say it went well.
mama to callum (april 8,07) and everett (sept 24,09) - blessed to be married to my life's love since '98.
Your son has 1 1/2 years to mature and become independent before going off full day. It's amazing how that time really changes kids. At 3 1/2, I don't know how my ds1 would've done at daycare, not well probably, he's always been very clingy and super attached to me...by the time he was 4ish, I was able to leave him in a drop-in daycare for a few hours to get work done or work out. We homeschool now, but I have no problem leaving him at the drop in daycare - sometimes he'll tell me he doesn't want to go, but I explain to him that is something that needs to happen (dh and I have date night, etc), and he doesn't struggle. Also, he loves going to art camps or workshops and these are anywhere from 1 hour to 3 hours, and he has no problem doing that for over a year now (he just turned 6) staying there, following directions, etc. I'm sure he'd be ok with school...not necessarily that he'd love it, but he'd be ok without crying etc.