It could be that his sugar levels are on the high side before and/or during the night.
When sugar levels are high, the kidneys will take glucose out of the blood and will take glucose and water out of the blood to be passed out as urine.
It is common for higher sugar levels to necessitate trips to the toilet during the night. If your son is not waking up when his body needs to pass urine he will sadly wet the bed.
If your son is having higher sugar levels before bed or in the early morning, this may provide an explanation. Ideally sugar levels should be in the recommended range which may help to reduce the chances of a wet bed, but low sugar levels should also be avoided of course so you may need to find a safe balance as best you can.
I hope things begin to improve and I'm sure they will.
All the best
Ed