the transition from reception to year1 is much bigger than the transition from preschool/nursery to reception.
Even if your ds is achieving well in terms of his academic ability, he may not actually want to be achieving academically and may still want to be playing, something which he is getting much less chance to do now that he is in year1.
How was he in reception? Did he ever get upset going in then? or is this a new thing?
My ds went through a stage of crying in reception. He couldn't tell me why, he was perfectly happy once he went in, but cried every morning for about three weeks. There wasn't anything wrong, I came to the conclusion that it was just that the penny had dropped that this was it now for the foreseeable part of his life.
I would have a word with the teacher and try to establish how he is when he's actually there, also, would it be possible for you to go in as a parent helper one morning so that you could observe from a distance? Or would you being there unsettle him more?
What works for your ds wrt getting him over an emotional outburst? for my ds, cuddles and sympathy and pandering just makes it worse, so i've discovered over time that distraction and ignoring snaps him out of it far quicker because he doesn't have the audience, iyswim? Again it might be worth speaking to teacher/TA to agree a strategy for him going in without the associated crying.
Sometimes it's just a matter of breaking the cycle, and once they go in once without crying, they're fine.
When my ds went through this I offered bribes, threatened punishments, had long conversations about why he was crying (he couldn't tell me). Eventually his teacher told him on a thursday that if he came in crying the next morning then he wouldn't be getting golden time. i aided this slightly by going with him to the shop before school and buying biscuits for him to take in for golden time, and he ran in without so much as a backward glance and we have never looked back.
I'm not saying this is the issue with your ds, but it's something worth thinking about. Most children go through a stage of crying at school, and for most it really is something that they get over with the right support.