Yes, learning objectives should be differentiated so that all children can be challenged.
How is your son doing with the level of work - is he succeeding with each piece? Are all homework tasks being completed with a high level of success? What have his assessment scores been like this year?
Does the school website have a section that identifies what each subject covers in each year group? That is a good way to look at the 'big picture' and can inform any discussions you need to have with the head of department or teacher.
In terms of being constantly reminded that they're in one of the lower sets, that can be quite a knock to the confidence. It could be that the teacher isn't a great communicator and what they actually mean is 'we need to go back and secure some basics before we can move on to x, y and z'.
Definitely worth having the discussion, but you'll be doing yourself and your son a big favour to go in with facts, test scores....evidence basically. Asking for a meeting and saying something like 'the attitude of the teacher and TA needs to change' won't achieve anything - especially in a time with crazy staffing shortages and real recruitment difficulties.
Good luck. I was moved down when I started GCSEs, spent 2 years complaining that it was all too easy as I was at the top of the class and then barely scraped a C. Embarrassed much?! How I wish I'd put that energy into attempting some of the work of the top set, discussing with my teachers in a more meaningful way etc...