I am just NOT a fan of changing wet sheets in the middle of the night... at all! So, I'm not really sure of an answer for you since my DS is the same age, but I wanted to reply to at least say hello
It seems to me, that if your DS is wetting his diaper during the night (even if it's in the morning hours), then he is just not awake enough to be able to realize it and get to the bathroom in time; therefore, letting him wet the bed isn't going to make him learn. On the mornings that he is dry, I assume you take him to the bathroom immediately?
Once my DS was (finally!) potty trained, at almost 4 years old (ugh), DH insisted he wear underwear at night. I rolled my eyes, knowing it would be ME to clean everything up. Well, we had two incidents early on, and have not had any since then. Hallelujah, something went right for me
When DS does have to go in the middle of the night, it's 11 pm- midnight, but this is rare. So maybe, this is the way to go for you, too... maybe this is what he needs and you will only have a few incidents too. I know, I've contradicted my answers, so not much help, sorry!
Lots of luck to you, the potty training thing has just about killed me!