S.H.
I don't want to bum you out, but I have 2 brothers and both of them were 6 before they were potty trained thru the night...it is something about boys. I am suprised your doctor isn't aware boys take much longer to train thru the night than girls. Some boys do it fine but a lot of boys don't and that is why there are products on the market now for older children (good nights sleep underware). I think that pushing the issue and punishing are not the answer. Also with the alarm, the child actually has to start wetting before the alarm goes off and thus it can be hard for them to stop once it started.
What my mom says is that both boys slept so hard that they didn't wake up when their bodies were telling them they needed to go. Both of them admit that they would dream they were going to the bathroom many times that they did wet their beds. If you have ruled out all other medical explinations for his bed wetting this may be the problem (other medical conditions include constipation, genetics, hormone imbalance, adhd, ect). Here is a website that talks about bedwetting in older children http://www.patient.co.uk/showdoc/23068688/.
Don't get down about this and don't let it become a huge issue. I am sure your son is having as hard a time with this as you are...make sure he knows you love him and you are in this together!