I have an 8-year-old with some developmental issues that impact his planning capabilities and make complex games too frustrating for him, so I bought Hisss.
We liked the game, but it was a little too simple for the rest of us (and soon for him).
Soooooo we tried it tonight with this variation: each player has one tile in their "hand." When it is your turn, you draw another tile from the pile and you have a choice of which tile to play. This doesn't make this a high-level game, but provides some more choice to add strategy and interest.
We all had alot of fun with the game tonight - enough choices to keep us all engaged - enough luck that no one could take losing too personally!
That's an interesting variant. I might try it with my kids some time.
Just as a thought - maybe you could start on one of the Carcassonne variants. If at first you just play it like a puzzle, it is not much more complex than Hisss - really just matching 1-4 sides rather than 1-2. Then perhaps after mastering that you could gradually introduce other game elements.
It's great that you find games to be an enjoyable activity with your boy. I don't know anything about him at all beyond what you've said, but it may be that it could be a really good development exercise for him, and one that can be done in a safe, loving and fun environment.
Also, take a look at some of the lists for kids' games. I'm sure there are many that he would be able to play and would really enjoy.