Thank you for all these replies.
Many of you have suggested that my parents save for DS rather than get him presents. while this is a good idea in theory, it really won't work.
Simply because we have actually already had a protracted struggle over the Child Trust Fund because they wanted to deposit £200 a month in it. We really strongly felt that it would be a terrible idea for DS to come into huge quanties of money at eighteen (and be able to control it at sixteen) and vetoed it.
The compromise we came up with was that my parents opened a separate account in DS' name and saved the money in there to be given to him when he really needed it for university or a deposit on a house or setting up a business or whatever.
Despite this, they still - in addition to the above arrangement - frequently give DS £100 at birthdays / Christmas / Easter / when he does a fart to go into his Child Trust Fund (which he does have of course, we're not anti him having money just not thousands and thousands at a very young age). Grrrrrr! We wanted the CTF to be something he could see was being build up gradually with small gifts from his parents, relatives etc for him to use as he sees fit when he is 18!
So, we're actually trying to stem the flow of money already - a suggestion that they give him more would land us back at the beginning!
Books is a fab suggestion. Only hitch is that my parents don't see books as presents, more as necessities and they do bring him books almost every week. I am a huge bookworm myself - thanks in part to my parents I imagine - and I understand this attitude. DS has -literally - hundreds of books but he sees them as being like food or clothes. As do I.
Part of the appeal of cars / stickers / trikes etc for my parents is that DS knows that they are treats (albeit ones he unfortunately takes for granted now) and they love seeing him enjoying and playing with a treat they have brought him. DS loves to read and be read to but neither he, my parents (or I, I suppose) view books as a treat, more as a daily activity.
But...many of you have told me just to back off and let DS and his grandparents enjoy their relationship and this may well be the most sensible option (as well as the path of least resistance).
I am very grateful that they love him so much and I don't want to interfere but I do have niggling doubts about the values that this constant providing of money and gifts will instill. I do also worry that he will grow up with the expectation that Granny and Grandad will always provide whatever he wants, regardless of whether his parents have denied it.
. Sorry, I do know that there are bigger things to fret about, and I do keep re-reading your responses to try to take on board your suggestions!