Your sons behavior is pretty typical for someone his age. According to many developmental psychology texts and other baby books, both of your children are still in a very self-focuses age where they may not really play together, but play separately near each other. It is a normal part of child development. It is important to still promote caring & sharing behavior, but it may take a while before it really sinks in. I have a six-year old step-son and a 2 year old daughter (about to be 3 in June). My step-son did have a harder time learning to share than my daughter. After all, he was basically an only child for 3.5 years until my daughter was born, and then sharing still wasn't an issue until she was about a year or more. My daughter on the other hand, being the youngest, has always had an older sibling (my step-son) and has never experienced the concept of having things all to herself. I will admit that if one child takes a toy away from the other, I return the toy to the child who originally had the toy, explaining that we do not take things away from each other, we must first ask and receive permission, and then find something else for the other child to play with. It is a long and arduous process. So, time, patience, encouragement and you can expect them to play individually for a while yet.
I hope that at least some part of that was helpful.
Sincerely,
P.