I used to be a secondary teacher andthey unfortunately streamed right down to the form groups as well as subject areas,. My form group was the bottom set (of 8). Every child was on the special needs register, but then over 30% of the school was.
But there was little distinction between children with learning difficulties, emotional problems, etc. They were commonly reagrded by EVERYONE - themselves, their peers and the teachers as a problem group (which they were) and totally thick - their opinion & all the other kids', who used to say much worse comments all the time. Being moved into the group was seen as a punishment, and unfortunately some teacher of middle bands who'd had enough of poor behaviour would call for a pupil to be put in the group.
Once in, it was extremely unusual for a child to be moved up.
I really believe the school I taught at was the worst example of educating children I have ever seen, and nothing on this earth would make me sent my kids there. Nevertheless, I would be really concerned if my child was being placed in a bottom set.
I'd want to know-
How flexible were the groups? ie What chance is there for him to move up? Would it only be possible once a year (ie at the end of tests & reports?)
What support is available both within the group to help him improve, and in the new group for hopefully when he gets moved up?
What the specifiac problems of him and the other kids in the group are etc etc.
Having said that, I also worked with the bottom set in another school, and it was a wonderful place. Other kids used to beg to go to the class, and had to make do with break and lunch times, as the saintly teacher used to let all and sundry in for these times.