A.M.
Your son actually sounds A LOT like my first son. My son was about the same size and very, very active. He actually started crawling and pulling up at about 5 1/2 months old. And like your's slept about 17-18 hrs a day. I was concerned and so asked our pediatrician about it. He told me not to worry because a.) all babies are different b.) he was active and so he needed the sleep c.) he developmentally ahead or on track in all areas and d.) he said that sleep patterns tend to run in a family, like it's genetic. My dr. told me to find out if either my husband or I were "sleepers" as babies and that would answer it. I slept a lot as a child (and still do). So I stopped worrying about and he has turend out as a normal healthy kid.
I just now wish my 6 month old son would sleep like his brother. I can only get about 10:30-7:30 for him and about 3 thirty minute naps during the day. Ugghhh! But you know what...he's developing fine as well with his "lack" of sleep. Go figure!