Big hug JKS.
As an ex-teacher, I would say firstly that I always said something positive bout the child, probably three or four things. This was more to keep me sane and build cooperation with parents, so of course I think she should have done that.
I also tried to flag up any problems in school books so parents would not get nasty shocks at parent's evening.
I also think it is the teacher's job to teach your child.
I certainly would not pile on more 'school' work. I am pretty anti-homework in general.
I thought those suggestions about developing motor skills were great. Does he like arty things? Maybe something like spirograph too if he does ?
I'm sure he's just fine. IME children and people do develop at different times. In my case, you would never have picked me out of the crowd at school. I just didnt like it at all. University was different and I soared. Others soar at school and then lose it when they lose that structure.There will be at least four girls in his class with perfect handwriting who will write exceedingly tedious stories for pages and pages, get good marks, praise and no problems who are in fact pretty average, believe me. You my feel madly envious of the approbation they get, but don't. You know they are in every class. You know and love your little boy and know how to support him to be what he wants to be, however long it takes for him to hit his groove academically or in any other way.
More than that though, you need a big hug because he's your lovely boy and you don't need to be made to feel worried about him. He's coped brilliantly with his brother's treatment and that speaks volumes.