We’ve had loads of stuff out of Kids Army Shop and it’s been perfect. Much of it went into redecorating his room too.
Swat vest, army vest, we’re really lucky we have a military surplus shop near us with some things in that are appropriate for kids.
Tanks, all tanks, large tanks, the tankier the better! Ditto helicopters.
Camo net, camo tent, den, fantasy box fort.
Ammo tins for ‘secret stuff’ - KAS do a metal one which is small enough for a six year old to carry but contains a fair bit of ‘secret stuff’. Also, having important things to stash means they don’t leave them on the floor where I stand on them in the dark 
Dog tags.
Nerf guns (and if a small six, a cool helmet or mask also is a blessing because invariably someone gets hit in the eye). We’ve mostly avoided noisy guns by having nerf guns. Be warned the rocket launcher type ones do, actually hurt.
Army men. Army men with parachutes.
Swat lego men - we got these off ebay I’m sure they’re knock offs.
Playmobil do an AMAZING swat boat, which you can also get a battery waterproof propeller for about £6. Best present last Christmas.
We also got some RAF plaques and medals and ‘bits’ which were great for teaching him about distant relations, their service and their contribution to the Far East campaigns.