I think you're overthinking this.
Just take away the directions and encourage him to free build. He will have lots of interesting pieces. You can get add on kits that show have different pieces that allow you to make even more moving things with what you have. Right now it sounds like your son is at the beginning stage of lego building. Are you getting kits that are too hard for him? There is a learning curve but it's like any other thing--once a child learns the basics of (fingerknitting/beading/working with clay/a game/lego building) then the kits go together much faster because they only need to pay super close attention to things that are unusual--and then ONLY if they WISH to make the final project look like what's on the box.
I see a lot of parents end up a) micromanaging the process and frustrating their kid, b) getting kits when the kid is not ready for kits, or c) getting inappropriate kits for their kid or d) giving the child the impression that they MUST build exactly what's in the kit or that they can't mix and match.
Your kiddo is only 6. To be honest, I would just try to keep it simple. It would also be kind to invest some time to teach him how to read the directions for himself, and to help him work out a system (some kids sort, some don't, ect.) for when he wants to assemble a kit. And to encourage him to always use the pieces as he wishes for free play and that if he's getting frustrated that's okay and normal (and part of learning how to build bigger projects, even grownups sometimes have to redo stuff!). Some kids really do need to be told that freebuilding (even with kit parts) is great and fun and to be encouraged (you might have to mind what dad and grandpa say to him, are they hyperventilating about him mixing pieces?).
My boys are lego fiends. They're 7, and regularly put together high level kits--but they've also been lego building for years, and the first couple of years were freehand and/or through workshops where they were given projects (but not from a kit). I too introduced kits too soon, and it was disastrous, so I just put them away or gave them to them as "ideas" but told them to just build whatever they wanted. These days they like to put together the kit as it's supposed to be, then take it apart and then reassemble with other things (the Hogwarts Castle and AT-AT they got as Christmas gifts were built and then taken apart and now they've combined the pieces into what hubby and I call The Borg Castle because it's all mixed together as one structure with a bunch of varied lego people all mixed up inside it. :D).
Legos haven't become anything more than what you make of them. A lot of folks find enjoyment from the kits, it's almost like model airplanes or whatever. Others love freebuilding. Many people like to do both and then merge. If you guys have anything like a BrickCon around you, it might be fun to take your son, so he can see the creative ways that other kids, teens, and adults have put together set pieces in creative ways. Or you can probably find a ton of pics online.
Relax. Legos are still fun. If sets stress you out, then either don't buy them, or repackage them, and ask your loved ones to do the same. At his age, it's probably more cost-effective to buy a treasure trove of mixed up pieces from ebay/craigslist/garage sale or get a baggie of mix and match pieces at the lego store than a bunch of kits unless your son enjoys them and you're willing to invest some time in helping him learn how to do it. The lego police will not come and arrest you if you switch Luke Skywalker's head with Ron Weasley's or put a bunch of car wheels on top of a spaceship.
ETA: Yikes, for some reason 6 was the age stuck in my head. I see your kiddo is only 5. My boys were given their first pile of legos at 4, they weren't really ready for kits until 6ish and had done some freebuilding projects both at home and via a really cool camp that our parks and rec offered. Once they hit 6.5 they could assemble simple kits without assistance, and rapidly went up to more complicated kits but they still enjoy simple ones as well. I would use the "age guides" as less about what age the kid should be and more an indication of complexity. And as always be ready to advocate for freebuilding if other adults tell him he needs to do it "right!"