He is probably just being lazy, but there are several things that could affect his development that you should have checked out.
If he has good muscle tone and his joints are fine it could be a cognitive deficit which he could outgrow or may need physical or occupational therapy for.
Have you had his vision checked? It is a possibility that he has vision problems, my nephew had some vision problems (he was a preemie and had many problems) and wasn't walking, then his eyes were fixed (his required surgery) and he was walking like a pro after that.
Check out this developmental site and if you have concerns that your child is not developing normally talk to a pediatrician or child development specialist about it.
http://www.thenewparentsguide.com/baby-development-main.htm
Remember all children develop differently and it is only after several months of missing milestones that you should truely worry, but also remember that catching a problem earlier is better. No question is dumb, and no concern is unwarranted, if your pediatrician is shrugging it off and not listening to your concerns, find a new one.
Also if your baby was a preemie don't forget to subtract the number of weeks early he was as this would be his actual age, (so if your baby was 4 weeks early he would be considered 15 months instead of 16.)
Good luck. :)