take heart. All children are individual and meet their various milestones and development levels when they are ready.
You may wish to discuss the issue with your son's MD when next you see them (no need to make an appointment though if they accept email don't hesitate to open a conversation now). The MD can ask a few questions to rule out the likely hood of an underlying medical condition.
My very nearly 8 year old son still has nightime wetting issues. We have taken several approaches while always trying to be very supportive. It is not uncommon and usually not something the child can control.
I do recommend www.bedwettingstore.com which have some great tools to make life a little easier and if needed to help with the transition to dryness. They have several books on the subject as well. Waking Up Dry by Dr Howard Bennett is fairly friendly and straight forward book with sections for parents and sections for kids. It helps everyone understand what is going on and provides some concrete exercises to try as well as discussing various approaches.
From a personal perspective I would hesitate to start a young child on an alarm. We allowed our very enthusiastic to try this 6 year old (at the time) to choose and alarm and try it. He really did not like it so we stopped and are waiting to try again soonish now that he has matured somewhat.
There are pros and cons to any approach. I would recommend a low key/no stress approach without blame.
Often there is a familial component though it need not be one of the parents who had this problem. I discovered my mother had night time wetting issues until she was about 10. We are hoping that by providing support and understanding and helping him develope a plan when he is ready that our son will transition to dry nights within another year.
He has a camping trip with school approaching in May. This is such a common problem that the school deals with it every year and makes sure it does not become an embarresment for any kid. Most simply use night time pull ups under their pj's and have plastic bags available to wrap and dispose properly.
It may be necessary for you to speak with your older son and make sure no teasing and no disclosure to others is accepted.
I wish you well and hope you can relax and accept each of the differences your three children are bound to exhibit. :)