Bed wetting, or enuresis is common among children, sometimes up to 5 years of age. Most children do not have any physical abnormalities that can be "fixed", and almost all will benefit from behavioral modification. A child's bladder grows at about 1 oz per year of age and you can actually try to determine your child's bladder size by asking him to void into a cup after holding to capacity. Do this a few times to get an idea of how large or small your child's bladder is. Most children with bed wetting will overcome this problem between the age of 6-10, and treatments can take 6-12 months to work. But you should be reminded that most, if not all children will get over this with or without treatment.
Some of these behavioral modifications are simple yet can be extremely effective. For example, have your child 1) take more fluid during the day and less fluid close to bed time. 2) Pep talk to your child about staying dry i.e. Reminds him about how a full bladder feels; pretend his bladder is full and is saying "get up before it is too late." 3) Next, have your child self awakening. If this fails, try setting up an alarm clock for 3-4 hours after falling asleep. If both fails, then you could try parent-awakening.
If your child does not have any improvement despite strong effort for 3-6 months then you should see a pediatrician