Well, I'm not the boss of you, so don't change your ways just because I say so. I was just curious, although I do find it a bit annoying.

my son loves cartoonish graphics. thanks for the heads up about a new broken sword.

It's not particularly cartoonish, long, 2D or even point-and-click, but please consider Portal 2. The co-op campaign takes something like four or five hours to finish, but even so I would recommend it.

Try Discworld.

Without a walkthrough, it could easily take you 100+ hours to solve those insane puzzles.

How do you rate the length of a point&click adventure game? By how many mouse clicks minimum it takes to finish the game?

I think I've used over one million mouse clicks with some adventure games, or at least it felt so.

A bit more info:
https://www.revolution.co.uk/games/bs5/
http://www.kickstarter.com/projects/165500047/broken-sword-the-serpents-curse-adventure (funding ended on Sep 22, 2012 with almost twice their original goal)

Broken Sword is not that long, but I would definitely recommend it. The graphics are gorgeous, and if your son wants to continue the adventure, you can just get the second part, they are pretty cheap. And I think that they're pretty boy-ish adventures, very indiana jones - esque.

Also, do keep bolding your text, it makes it easier to find what exactly you have said :D

From GOG catalogue the longest 2D cartoonish game... Deponia, maybe?
Else:
Grim Fandango took me some time to finish for the first time (due to several stucks) - a word of warning: it's not 2D and it's not point-and-click. Still on my list of top 3 games ever made.
Feeble files were long, but again - it has one part that's insanely hard and not-that-fun (I'm looking at you, gambling club).
Also, you can try entire series: Monkey Island, Sierra's Quests, Larry - if you play them in a row, it probably can take you around 100 hours.

Try getting the Space Quest 1 - 6 collection and play them back-to-back? Same for some of the other Sierra quest games. As long as you're having fun, you might not care that you're stopping one game and continuing with another, plus they'll look similar and have similar characters, etc.

Try Gilbert Goodmate and the Mushroom of Phungoria. It is 2d cartoonish and depending on the age of your son you should enjoy it.

Tass Times in Tone Town was long but it was also older than the hills and had zero replayability.

100+ hours is never going to happen in a point-and-click adventure. Maybe if you get a whole series like Myst and count them as a single entity, maybe, but an individual game? Nope.

As for cartoony 2D graphics, anything by Wadjet Eye should fit the bill. They all use hand drawn sprites and backgrounds, and the stories are quite enjoyable. Personally, I'd recommend you start with the Blackwell series. Short games, but there's four of them with more to come, and the characters are a hell of a lot of fun.

Not 100 hours, but the first two Simon the Sorcerer games are 2D and cartoony. If your son's on the young side---i.e. too young for things like Waxworks or Leisure Suit Larry--- they'd make a good, relatively cheap choice. There's always the free point-and-clicks as well.

Definitely not Fullthrotlle for the length .... however if you want to teach some (more?) badassness to your kid, go wishlist to kick up some wishes.

Was going to recommend playing all 3 series of Sam and Max at once until I remembered you said 2d...

Try some of Telltale's games anyway, their 3d art is still pretty cartoony your son could enjoy it.