A baby usually starts making his/her first step between 11 and 15 months. That being said, it varies for every child and some may only start walking at 18 months.
Typically, from 9 months onwards, babies would pull themselves up to a standing position with the help of a sturdy object. From then on, babies will attempt to walk while holding onto something.
You can try playing these games with your baby to help him strengthen the muscles needed for walking:
http://www.mom365.com/baby/baby-development/5-walking-games/
Do consult your doctor if your son is still not walking by 18 months.