Kel
Go and see a private OT; they will assess your son, they may not give you a diagnosis, but they will be able to give you some strategies and advice which may help him build up his core stability and help with his gross and minor motor controls.
Make sure any OT you use has experience of working with children with learning difficulties specifically Dyspraxia.
They may be able to advise on pencil grips etc, but if as you say 'his grip isn't there', personally I think you are jumping too many stages ahead by focussing on him writing. My son had a similar difficulty when younger, couldn't hold chunky crayons correctly, could only make emergent marks on paper. He still has an akward grip with pens, but he has learnt to write.
Unfortunately you will have to get used to schools making implications about your parenting and/or your child being lazy. We used to get 'well hes an August baby and a boy, what do you expect, they're always behind'(not true). It is an effective strategy used by schools to justify your childs lack of progression by focussing the problem back on the family, rather than the lack of effective support in school. Grow a thick skin and go with your instinct. Try not to let it knock your self-esteem.
Buy these books, it may give you an idea of what to expect, things to try etc but remember not every child with Dyspraxia is the same, what works for one may not work for another etc.
Dyspraxia by Amanda Kirby
Developmental Dyspraxia by Madeline Portwood