Whowillsave:
Look I think as others have said your son sounds like he's doing very well - some kids are blithely unaware and others are really upset by being moved tables (sometimes just because they're moved away from friends).
Ability tables are a nightmare for parents & teachers. As a teacher if you see 8 kids but only have a table for 6 - you shouldn't (I feel at least) - but some do exclude 2 because the table only has 6 chairs. Some schools don't place children on tables, others do but not necessarily by ability - other schools really struggle not to convey to children/ parents what the true ability level of children is. Certainly my kids knew who was on 'top table' from very young ages.
I think it depends on the school - some schools only give challenging material to their top table - other schools give all pupils access to difficult maths problems. (It's a pedagogical divide - some teachers feel that challenging maths that pupils struggle with will be upsetting, possibly putting them off maths - only add when you're ready kind of thing (which at our school at least necessitated learning that at home, proving it at school & then you got such work) - others feel that you shouldn't presume anything, should be primarily teaching maths skills in school - and give an open invitation to all children to try a difficult maths problem which they can opt to take up or not as they chose).
As a parent it can be concerning - why has my child been moved down a table? Are they struggling? Are they not paying attention? Are they falling behind? - all of which are real worries but schools are resistant to explain that your child is ranked 8th in the class and there are 7 more able kids but only 6 places at the top maths table, etc....
So my view is this - if you feel that your child is struggling, if for you it's important they're top table (and that's a very individual thing), if you feel where they are now the maths is too easy - do more at home.
It's hard to know whether your DS is in a school where other parents are doing more at home - you read all sorts here on MN about parents hiring tutors in Y1 in some areas because competition for grammar school places is so fierce. It may just be a particularly bright year group he's in the midst of. It really is hard to say without knowing your specific situation - but you do have options.
If you feel your child is missing out/ not getting concepts - check that against the programme for study for KS1/ KS2 in maths: www.gov.uk/government/uploads/system/uploads/attachment_data/file/335158/PRIMARY_national_curriculum_-_Mathematics_220714.pdf
Doing more at home may be the solution for you. No it isn't how it should be - but I've found schools tend to just do what they have to and no more. There are options out there - there are all sorts of free on-line games (Woodlands Junior Maths Zone being one I'd highly recommend)/ on-line maths tutorials and workbooks available.
HTH