First of all, try not to worry yet. Every child develops differently. He may just be working on his own timeline. Also, sometimes, second children don't feel as big a need to do different activities because of their older siblings. My cousins daughter did not speak for the longest time because her older sister was always talking for her. =)
That being said, I would consult the doctor and have him checked. He may need some assistance in getting him to reach the developmental norms right now. That does NOT mean there is anything wrong with him or that he will have a condition for the rest of his life. What it would mean is that he MAY need a little help now. My friends little boy needed some help with his early developmental milestones. He is a very intelligent and active little boy now.
There could be any number of reasons why he may not be developing on the same timeline as your other son did. The biggest reason of all could simply be that he is a different individual who wants to do things when he wants to do them. Who knows, you may find that he develops faster than your first son in other ways.
Try not to worry. I know that's a lot like telling someone try not to breathe but you are already doing everything you can and you don't know there is anything to worry about yet. =)