We have a vast selection of Christmas stories but I can't really think of many we own which I would describe as 'funny'. We very much prefer the 'charming' tales.
Only one that really springs to mind is Santa Selfie by Peter Bently. I use to own one called The Sheep who Saved Christmas too which was quite funny but I can't remember who wrote it.
Maybe the Sue Hendra ones:- Norman the Slug who Saved Christmas, Supertato - Evil Pea Rules, Snowball (more wintery than Christmassy), Oh Christmas Tree. He might be a bit young for those though if he's 6, as they're aimed at pre-schools, apart from maybe Oh Christmas Tree, where the humour is quite sophisticated (personally I'm not a fan of that one, but he might enjoy it).
There's also Aliens Love Pantaclaus, although this is also aimed more at under 5s I feel.
Tom Fletcher and Dougie Poynter have a new dinosaur that popped book out too called The Dinosaur who Pooped a Reindeer. I'm not a big fan of the series but it's in line with a 6 year old's humour. Tom Fletcher also does that interactive 'There's a in my book' series which he may find funny. I'm sure one is about an elf?
There's a series about 'my bum' which has a Christmassy version I've seen in the shops (something like 'My new bum is so Christmassy). We don't own any of them, as I found the original 'I need a New Bum' to be awful when I flicked through it once but if he loves Wonkey Donkey they might appeal as it's the same sort of 'humour'.
The Greg the Sausage Roll books are truly dreadful imo (reads like a 6 year old actually wrote it) but I guess they are intended to be funny so he might like them.
There's another I've seen called Granny Got a Spout Stuck up her Sneezer. I've never read it, but I assume from the title it is intended to be a humorous read.