Don't worry about it. Your doctor is right and you can't compare your son to your daughter. They are to separate and completely different people. It's so easy to do that when we're parent, isn't it? Lol. As moms, we just worry too much.
My first two were saying complete sentences by the time they were 18 months. My youngest, who is two now has only just begun to really talk.
Have you tried baby signs with him? That helped my youngest communicate until she could verbalize. Imagine how frustrated he must be, knowing what he wants, telling you and you not being able to understand. The baby signs help my littlest one not to be so frustrated because she was able to tell me what she needed and I could understand...a lot less tantrums. :D
She could even tell me when she needed a change. Try it, it may help.