I work in Early Intervention and you had mentioned he was a late sitter and crawler. Crawling isn't really a part of the developmental milestones as is rolling over, sitting up, talking, walking, etc. Have you noticed any other delays? Is he babbling? Can he transfer objects from one hand to another without fumbling? When holding something, does he bring it to his mid-line position (middle of his body)? Does he try to climb by putting one leg in the air (I know you mentioned some balance issues so does he try before falling?) Can he stack blocks?
I ask you all these questions because it could be several things going on. I am not a doctor, but have worked with children from birth - school age for several years and as a special education teacher in the school district for 10 years. If you suspect a problem definitely bring the issues up with your doctor, but most states allow testing done without cost to parents. In New York, early intervention is handled by each county and parents can call without the pediatrician if they suspect a problem.
I am not saying your DS has a problem, couldn't do that just by what you have said. If you think there is a problem most likely it is minor.
It could be:
Fluid in the inner ears (offsets equilibrium causing imbalance) A doctor would have to look and do a tempanogram (Sp?) test. Very harmless and can be done in the ped's office.
Low tone which causes delays. Physical therapy can help if it is found necessary. Many cases they encourage more physical types of play like the playground, gym classes for toddlers, etc.
Maturing on his own terms which simply means he will walk when he is ready to walk. Nothing really wrong with the child. COuld be working on language development or problem solving which take up his time leaving little energy for the physical stuff.
BTW - If your DS is your first child, this is common. Second children do tend to walk a bit earlier. Big incentive to try and keep up with older siblings.
Average age for children to walk is 10 mos - 18 mos. 19 or 20 mos is only slightly behind.
One last thought- your DS might surprise you and not walk but run his first steps!
