You will need to pick your battles. The old teacher was great and the new teacher has stepped into very large shoes and will have 29ish other children to learn about mid-year. The previous teacher was clearly differentiating well for your son, but many teachers, even those that would like to challenge a brighter child, struggle to find much time at all to keep an exceptional child fully challenged, when they are dealing with an already crazy workload.
So, two examples you’ve given are two of the easy ones to address. I posted on MN once about spellings, and was told that unless I wanted my daughter to enter spelling competitions I should consider myself lucky that spellings are easy for her, and spend that time productively on other enriching things. I have taken that advice! We simply encourage wider reading, discuss unusual words when encountered, occasionally randomly test spelling for ‘fun’. Irvine’s advice about discussing synonyms etc is great too.
Reading. We’ve certainly wasted plenty of time worrying about the books DD brings home from school! If the book is very easy, make a suitable note in the reading diary, take him to the library and encourage him to read a wide variety of other books, and note every single one in the reading diary. You can choose to fight for him to be moved up the book bands, or you can take matters into your own hands. The latter is easier and much more fun.
In my mind the thing to watch for is that he’s stimulated in the classroom, that the teacher is encouraging that critical thinking and expects more/better from him, and that he is keen to meet/exceed these high expectations. In Y1 we struggled with this: DD would come home saying that once she’d finished her work she helped person X, then person Y, then person Z, then read her book. When I spoke to the teacher she said D was ‘so helpful’ and ‘I really must put a learning plan in place for her’ and such platitudes, but not a great deal happened. Still, DD was happy enough and we didn’t escalate further. The teacher tried a bit (bought ‘special books’ etc) but it wasn’t great.
In Y2 it is so much better. The teacher just seems to know how to stretch DD. She almost never ‘just reads’ but the tasks are more challenging, and the teacher expects a better level, eg of written work, so DD is kept busy. Her books are full of little extension activities which the teacher has set for her. Our impression is that the teacher is experienced and easily able to differentiate. Maybe not perfectly all the time, but sufficiently to keep DD busy and challenged.
Is your DS confident to speak to the teacher if the work is too easy? In Y1 when they were dictating sentences we encouraged DD to ask for a harder one. We encouraged her to focus hard and work quickly even if the task was easy, and then as soon as she was finished to ask for something else to do. Basically to be very proactive, but politely so. DD is confident and a bit of a goody-two-shoes so this was quite easy for her, and it did mean the teacher noticed! We also encouraged DD to set her own harder challenges, eg in maths when she’d finished what they were doing, we said to try bigger numbers etc. and write them all down. DD found it fun and the teacher never complained (to us). It does take a certain personality to do this though!
If your DS is unhappy or switching off, that’s the battle to have, IMO. That can’t be ignored. It’s easier, however, to do the positive things above, than argue with the teacher that they’re not stretching your child.