When you child is close to age two, try potty training him. Some children may be ready at age eighteen months and others not until around age three. 

Notice what time of day your child has his bowel movements. 
Buy a plasic potty training potty for your child to use.
Explain to your child that big people go in the potty instead of in a diaper.
Have your child sit on the potty during his normal bowel movement time each day.  Then every 1/2 hour to one hour or more have him sit on the potty to see if he has to urinate.  Ask him often if he does have to go to the potty. 
Teach him how to pull down his pants and pull them back up.
Praise your child when he goes in the potty.  Tell him what a good job he has done and brag him up.  You could even offer a reward each time to encourage him.

There are also books on the subject.  Here are some: