When I first played a game from the Worms series, I struggled a lot, but so I did in the begging with everything: Quake, Hercules, Neighbours From Hell. So, I'll mark it down, since I haven't tried it yet, and the game is a lot of fun.

Lilly Looking Through

Defender's Quest

Bastion (in "No-Sweat" mode)

TripleTown

Well, I can see that LEGO Marvel Super Heroes is the highest rated Lego game on Steam, and so far, a few people have recommended the Lego games, I'll add it to the list alongside Batman.

Hexcells looks great. For a while I was thinking about adventure games, but after Machinarium and Botanicula I lost hope for any other. I also tried different platforming/puzzle games, such as: Trine and Braid, but in the end, he watched a let's play of Trine 2.
I also played the Prince of Persia games when I was young and had very little difficulty to finish them. And a controller does sound more simple to use, added.

Lilly Looking Through
Perfect, and now I'm interested in this one as well.
Defender's Quest
This one seems a bit overwhelming. But I'll give it a go as well.
Bastion (in "No-Sweat" mode)
Gonna check the easy mode soon.
TripleTown
This looks good.

Go find an old I.M. Meen CD.

Oooh...I.M. Meen was a great game. Spent many months on it years ago, still have the CD, I think, but with 30+ levels of increasing difficulty (that I never finished), not sure I'd call it easy :D.

The original ZORK and similar suited me wonderfully at 6, 7, 8, 9 years old.

They are GREAT for increasing reading skills, problem solving, and expanding the imagination, I even recommend that he learn to make his own maps rather than use maps that have alread been made, it's really great brain exercise.

try
armorgames.com
kongregate.com
There are plethora of flash games short enough and simple enough to suite for healthy doze of gaming.
Good explorative platformers, second part even without enemies.
http://armorgames.com/play/4225/william-and-sly
http://armorgames.com/play/12754/william-and-sly-2
plenty of ccg, my favourite
http://www.kongregate.com/games/nulllvoid/hd-xyth

<span class="bold">Gomo</span> might be worth a look. It's a very easy adventure with cartoon style graphics. It can be finished in less than 2 hours so i'd recommend getting it in a sale or bundle.

That may not say much. I read as good as nothing when I was a kid until I was into my mid/late teens, and now I tend to read more than most people I know.

Brothers: A Tale of Two Sons worked out well as a co-op game with my 7 year old daughter. It can probably work just as well single-player, because it's not a stressful game, even though the controls are a bit complex.

I can also suggest Marvel Super Hero Squad Online. It's a free to play kid MMO, and it's likely than an 8 year old will be familiar with the Marvel universe. I personally don't like it that because it's for kids it's distilled into a low-text game that focuses on simple combat, but I do think it has kid appeal.

A couple games with 0-difficulty modes:

Icewind Dale: Enhanced Edition has a Story difficulty, in which you basically can't lose. I believe Baldur's Gate: EE and BG2: EE do as well. (Incidentally, the original Baldur's Gate 2 has the unused "killsw01" weapon, which is only available by cheating and makes the game absurdly easy.) There is a lot of reading involved in these games, and that may be too much for an 8 year old.

Dust: An Elysian Tail is a platformer with RPG elements, but if you set the difficulty to Casual, it becomes impossible to lose (if you die, you are instantly revived with full health), plus you get infinite dust storm, which is fun. All the dialogue here is voice acted, so the need to read isn't so much of a problem. I would seriously consider this one.

PixelJunk Shooter is a very satisfying game with easy controls and not complicated levels. Sure, sometimes things can get a bit tricky but your friend can co-op it with his son if they have a controller. I had a lot of fun playing this game and can wholeheartedly recommend it.

Has anyone linked to this thread yet?
Kid-friendly games help

From what I remember, Steamworld Dig was pretty easy and fun, even though it's real-time. The only thing close to a challenge it posed was the boss fight at the end.

Anyway, in general it sounds like puzzle and adventure games are more up his alley? Maybe Puzzle Agent? World of Goo? Unmechanical?

Still have issues in Icewind Dale though, can't kill a "Shadow", can kill lesser shadows.
Weapons are ineffective or whatever.
Not sure what I'm supposed to do.

Maybe an 8 year old would be able to figure it out. :P