It may be the angle he is sitting at or positioning his paper, or how tightly he holds the pencil or how hard he is or isn’t pressing, rather than the actual grip.
Try activities/ games to develop fine motor skills - plastercine, playdough (try making conductive playdough if he thinks the regular stuff is too babyish) - loads of other ideas online - and weight-bearing through his arms - wheelbarrow games, monkey bars, cycling.
You don’t mention it, but it is relevant whether he is left-handed, as left-handers push the pencil, rather than pull it, which uses different muscles, and may change their hand angle to try to be able to see what they are writing.
Speak to the school Senco; they may have (or be able get) different options he could try - pencil grips, weighted pencils, angled writing slopes, finger gym exercises. If his difficulties are beyond what they can support with, they (or you, via GP - it depends on your local area) can refer him to an occupational therapist.