Okay. I hear what you're saying about independent/imaginative play and him being more likely to enjoy doing Lego with instructions.
I totally see why you'd expect him to be able to manage independent imaginative play at the age of 6, but it sounds like that's not his thing. So, might there be changes you guys could make so you can set it up so he's more likely to have a good time?
I'm thinking:
If he enjoys imaginative play when you are with him, you could set up some story boards/prompts?
Or look at things that can be built and rebuilt creatively but don't necessarily require stories? I'm thinking of brio railway but that's obviously aimed at younger children (though I found they enjoyed it for ages actually).
Or maybe there are custom Lego instructions to make different things out of the same kits?
Or maybe he would enjoy reading books?
Or he might like listening to audiobooks while he's out in the garden learning to pogo stick/roller skate/scoot/whatever. You can get lots of free audiobooks from your library, usually via either the Libby app or the Borrowbox app.
My parents set me off on various learning curves - some easier in the summer but e.g. I decided I was going to learn to skip, so I practiced til I could skip a thousand times over the rope without tripping up. Presumably one could do the same thing with learning to juggle or basically any number of things like that. So I am thinking of things that don't have a winner or a loser, are mainly an individual activity (not too much co-operation required) etc. I had a trapeze on a climbing frame and loved practicing summersaults etc.
I would also be considering options for craft projects so e.g. could you set him off with some ideas and then he does the making? Or he might get on better if the project has a purpose so e.g. making birthday cards or similar? Could even use stickers and things if he's not a fan of drawing.
You might already have tried all this stuff but I am just trying to think of options that might be different from what you're already trying.