My daughter did this at his age, too. And she's a girl! But she's always been a tummy or side sleeper, and she totally pees up front like a boy. We tried a LOT of things (we were cloth by day and could NOT find a good cloth choice for overnight for her). We also tried the diaper doubler inside the overnight diaper (we were able to get the diaper doublers at our local Wegman's). That seemed to help.
But what helped the most was creating a cut-off for liquid consumption at night. We stopped giving any fluids more than 1.5 hours before bedtime. That helped, too. But you need to know if that's right for you and your child. For us, we still considered it the bedtime drink, but if your son needs a drink right before bed in order to sleep, then it probably would not work for you. It's just another idea.
Good luck.
ETA: WOOL or FLEECE! Use a good quality wool (lanolized) or fleece sleeper or woolen (lanolized) or fleece pants - or even a woolen (lanolized) or fleece diaper cover, made for cloth diapers - over the diaper at night, in addition to the doubler, and you may find that even if the diaper is getting thoroughly soaked, his clothes and his sheet and bed will stay dry.