as the mother of a child with SN - I absolutely do not think a teacher's opinion is always right, rofl, in fact I'd question most of them if they said the sky was blue
however, early years teachers do know more about co-ordination development in young children than GPs do
support for children with a diagnosed problem is hard enough to get, schools do not waste resources on children with no issues, I've worked in schools that are having to ration paper, nevermind being able to afford to waste someone's time on a child who doesn't need it
joint exercises are in no way detrimental
neither is allowing a bit of investigation to see if there is a problem or not
neither is working on the floor and having lots of space to move around
which is pretty much the whole thread - and is in reaction to things you posted
it may well be that the teacher has at some point mixed him up with another child, or that your wife has the wrong end of the stick about how serious the teacher thought it was - but taking him out of a school because they offered him help seems a bit severe 
I genuinely hope that it is a complete misunderstanding and that he has no problems at all with his handwriting, firstly because obviously I would hope that for any child, but also because if he did have a problem if you refuse to acknowledge it until officially diagnosed as a major issue by a doctor it will be too late to do anything to resolve it