DS will be 6 in the next few weeks but I can't say it's difficult to know what to buy him. 6 is an amazing age for birthdays/Christmas as there are all sorts of amazing toys aimed at kids that age. My only problem is limiting myself from going overboard because I just love it all so much myself. Even with his birthday and Christmas to buy for, I tend to distribute a lot of what I want to give to my brothers, parents and FIL to give him too, so it's not all too much. This is the list of what I know he'll be getting from all of us for birthday and Christmas. In addition I'm making my own advent calendar filled with Imaginext and Superhero squad figures.
Hue HD stop motion animation kit.
Kano build your own computer kit, pi3 model+screen.
Gorilla Gym with swing, trapeze, rings and yoga swing.
Trash Wheels Junkyard set.
Hot Wheels, Track Builder bridge set. (Hot Wheels with Lego compatible pieces.)
Playskool
Spiderman playset
Imaginext
Wizard tower
Mummy guards
Serpent figure
2putties, Finster, Goldar, Zedd
Mini megazord + white dinozord
Mega Ice Dino
Wonder Woman invisible jet
Wonder Woman mech suit
Jurrasic Park Dr Sattler & Triceratops
R'as Al Ghul Ooze Pit
My Freeze Snowcat vehicle
The Incredibles Super Juniors
Ship playset
Underminer set
Extra figures
K'nex
Big Wheel
Roller Coaster
Lego
Several Scooby Doo kits
Lego watch, glasses and collection of minifigs
Basketball hoop + tennis net
Hexbug set
Sick Bricks
Pinball Machine
Stick and ball style magformers
Retro 80s Pac-man boardgame
Maze boardgame
Monster 500 cars
Wooden robot puzzle
Plastic twist puzzle
Minecraft cushions
Disney + Cars Top trumps
Batman mask and pjs
Invisible spy pen + uv light
Retro Mighty Max sets
Superheros Ooshies (pen toppers)
Books
2 Marvel Power Pack graphic novels
3 Marvel Adventures graphic novels
1 draw comics Marvel Way book
2 transformer books
Super puzzle story
Air craft book
Ben 10 book