My dd's school 'sets' for English and Maths, so - depending on who takes the set you are in, they could have 3 teachers every day, and then once a week the PPA teacher will take them, and of course, occasionally for illness or courses other teacher will take them. I think it's a positive in that they get more than one teacher, and it makes the transition to secondary a lot easier. However they know what is happening and the same thing happens each day, and the same teacher always takes them for maths, for example, so tracking, planning etc is all there. At various times, dd1 also had job shares, so that added to the numbers, but again, there was consistency. I think it can unsettle some children more than others, and the key to it is consistency though, which it doesn't sound as if your ds has.
My DD has one main teacher and <checks paperwork> six or seven other adults regularly in the classroom. Not counting supply teachers, lunch time supervisors etc. I must admit I saw this only as a good thing, in terms of her being exposed to different adults.
The prep school I was at had different teachers for every subject. It was quite nice actually - if you didn't gel with a particular teacher then at least you only had them for x hours a week rather than all day for a whole year.
As long as the teachers are capable of teaching the subject and as long as the lessons progress (ie they're not having to establish what the kids should be learning and what levels they are at in every lesson) then I don't really see any negatives.
DD (Y2) also has a training teacher in her class atm. And her school sets across the year, so she has the "other" teacher in her year group for some things. Plus she has a cover teacher who covers PPA and when her normal teacher is doing SLT things. And sometimes the TA takes her in a small group. And sometimes the "other" classes TA when they are working across the year group.
Actually writing that down, I'm wondering if her actual teacher ever teaches her! But, anyway, the point is that the school is making sure teaching is consistent and DD knows who is teaching her when and doesn't find it confusing. In fact I think she quite likes being taught by a number of different people - she has certainly made comments comparing the ways different teachers do things.
At the junior school she will probably go to the children have lots more lessons with specialist teachers.