As well as dysgraphia, you will find advice under Dyslexia and Dyspraxia.
Pencil grip is related to fine or gross motor skills.
You might find a forum for any of the above where other parents have posted tips which they found useful.
You can buy specially shaped pens and pencils, or pencil grips, such as this multi-pack, which would allow you to experiment with what works best.
http://www.letmelearn.co.uk/combipack-pencil-grips-p-573.html
I have a very awkward pencil grip, apply too much pressure, and had lots of difficulty as a child. My handwriting was always criticised and very messy.
Now I use fibre-tip pens with a comfi-grip and I have been complimented on my handwriting, so it does make a difference what you use. Ball-pens are always difficult for me, and round or skinny pencils/paintbrushes.
It isn't just handwriting, drawing and artwork need good control as well, so you can encourage practise with other activities besides writing, especially if your son enjoys them.
But do be patient, because the discomfort caused by trying to make your fingers work how you want them to is almost impossible to describe, but can be very frustrating, even as an adult. And I get twinges in my other hand and my toes at the same time, so don't be surprised at a comment such as "Writing makes my toes hurt." or any other apparently unconnected statement.
(It is also possible that he doesn't like the scratchy sound that some pencils make, especially blunt ones.)
You might find that a softer artist's pencil, eg a 2B or 3B, rather than a standard HB, allows him to make better marks without increasing pressure. All successes will boost his confidence. And stubby artist's crayons, because they are softer and easier to use than pencils, might be worth trying. A colouring book on his favourite topic might encourage him to use more pressure.
I hope that these ideas are useful. Good luck.