Hi AQ. My ds2 is 5.5, and still definitely in nappies: I'm in exactly the same boat as you, every now and then I leave the nappies off, but he is so clearly way off success, and it just results in mountains of laundry, including duvets. (He's usually in bed for 13 hrs, asleep for most of it.)
A friend recently told me I should persevere as she had with her dd ... turns out she had 2 or 3 sets of bedding to wash EVERY NIGHT through the summer for weeks and weeks, and even now still has 2 or 3 sets every week. NO WAY am I doing that.
Waterproof duvet protectors are horrid, rattly, sweaty things. It's like trying to sleep under a gigantic crisp packet! And you are very welcome to the one I have, but I doubt you want it after that recommendation
DS1 was very wet until he was almost 5, and then dried up literally overnight. Now he has the most fantastic amazing bladder control!!! I'm surprised ds2 hasn't made it yet, but I'm trying to be patient and wait, as I just can't face the alternative.
PS I thought your ds was older than mine.