It is not something he can control - DO NOT PUNISH HIM! My 5 year old also had the same problem - in that she had not achieved night time dryness. After a little internet research this is what I learned:
It is often many things combined that make this happen so could need a many sided approach.
One of the elements is a sleep disorder where they sleep to heavily and can sort of squirt the wee out in a sort of contraction related to the sleep disorder.
There are a variety of medications out there that are very helpful (you need to go to a doctor for this) but they have an 90% relapse rate when the medication is stopped but they are brilliant for sleepovers!
I used an alarm (Malem make a good one). This has worked a treat with my child who was also 5, however it is actually not recommended at this age as THEY ARE TOO YOUNG. They recommend from age 8 or so. But as I say it did work for my daughter. It requires (especially at this age) a lot of parental intervention. It work on the principle of teaching the child the feeling of needing to urinate and helping make this a trigger to wake up. As a parent, you then need to hear the alarm, wake the child up, take it to the toilet and then reset the alarm - at 5 they will not ever be able to reset the alarm so you will have to do this, but older children learn to do this for themselves in the night.
The biggest problem with the alarm system (which has a very high success rate) is that they are shockingly expensive between R1000 - R2000. I bought mine online from the UK slightly cheaper - but still an expense. I have heard of places that rent them out but have not found any in SA.
The most important thing that I found out tho is that is actually very, very common for 5 year olds not to be dry at night something like 15% of all children.
Hope this helps and give your child a big hug, he really doesn't mean to make all this work for you.