I have a DS who has just gone into year 3, when he went from reception to year 1 he did find it quite a jump up, the emphasis less on play based learning and more on actual work. He was a sad boy after school too, and it broke my heart also, he really noticed that 'others were better than him' but I soon nipped that it the bud and told him NO NO NO. He needed lots of TLC in the 1st half of year 1 and I foun not oo much emphasis on school work helped, like reading books he had chosen together that were too old for him or he could not read.
He was the same as your DS, found it hard to star with, but slowly and I mean very slowly he gott to grips with it, at his pace and I can't strongly say enough HIS PACE. Ignore what all the other kids are doing, hard to do, but just concentrate on your DS, they all all different and getting caught up on this, does you or him no good at all!
It is such early days now, just let him relax and be when he gets home, it the teacher has any concerns, she will talk to you soon enough. But if in a 3-4 weeks you still feel concerned go and arrange a meeting for a chat.
Year 2 was a great year for my DS (he was 7 and really found his stride, confidence, not so much on writing but we are on the right road), year 1 not so much, I think kids are like wine and develop their richness and strengths at different times and they need confidence and stability to go on the right path, and comparing and contrasting is a very bad idea. Don't get me wrong I do it still on bad days, but now have learnt to stop myself.
Play when he gets homes, let him watch TV, they need space and to zone out in year 1, especially as their brains are working overdrive.
Bless them and please don't worry. xx