I really really sympathise but surely your son is aware of his parental shortcomings - I mean you raised him better after all. Maybe there are some financial pressures or perhaps the parents are just plain exhausted - these are the usual reasons why the small extra cost and effort of veg and "proper pudding" becomes too great.
I have a similar issue with my GD. I blame the sin-in-law! When my GC has had too much sugar and E numbers she cannot sit still and concentrate, nor settle to eat a proper meal.
Are you able to offer any practical support? I try and have my GC to tea on a regular basis. Could you do some simple cooking with them when you visit? - maybe add toppings to a pizza (they can always pull off what they don't like, but it will give them the opportunity to try).
You could take a few sweet english apples with you (in season atm), peel and core them and cut into chunks and offer them round and eat some yourself. Apple is a good fruit for fruit and veg haters as it is not to wet/squishy/strong tasting so there's not much to take exception to.
My GC is a real square eyes but she is thrilled with the Peppa Pig comics which are very educational and offer lots of interactive adult and child learning and playing. We watch DVDs with her and we singalong with the songs, join in with the repeated phrases, invent silly dance/mime/action routines, before you know it the TV is just a background noise. (We do the same thing when we read stories and we keep the books under the TV - cunning eh? When she wants a second DVD we suggest a story instead and she usually agrees).
Find episodes of Bagpuss on You tube - it's ace for this age group. Build listening and communicating skills by asking "what do you think happens next" or ask them to tell you the story in their own words afterwards. Encourage the GC's to refer to the stories "that's like when Peppa Pig's brother went to her play school"
I alway comment favourably when the GC is better behaved and say what a nice time GC has had and DD has made the connection with sugar and diet generally and is really working hard not to "give in" and take the easy option. Just the sin-in law to convince now;)
Things you find stressful that other people don't notice.