Children can vary enormously. We have a GS who is the same age. Academically gifted, football mad, sociable, no problems at school, but he has yet to sleep away from home without a parent. He won't do sleep-overs, stay with us, nothing unless his mother is there and he still has meltdowns over trivial issues.
His parents are not over protective and his sister has had no problems at all. He has had a succession of minor health problems, and is having tests and examinations at the moment, but he is rarely off school.
We just think that his mind is way ahead of his age and this may be at the expense of his emotional development, but he will get there in the end.
Hildagard, I am sure something similar is happening with your DGS. As you say your relationship with your daughter is delicate. Do you think she is over-simplifying and over-dramatising what she has been told? Yes, your DGS has behavioural problems, which could have many causes, not necessarily ADHD, autism or anything else like that, and whoever did the assessment commented that emotionally he is young for his age, which, as several have said, not uncommon in a boy, the assessors probably did not say that emotionally he is a toddler.
I am with Bluebelle, I am sure the problem will resolve itself and if he does need some extra support the school will put this in place.