Mommies, help.Really worried about my son gaining weight too fast.`?

My 15 weeks old son weighs 16 pounds and measures little more than 25 inches(64 cms).He was 6 pounds and 19 inches at birth. I am really worried tht he's gaining weight too fast and might have problems later.He is exclusivly BF(only). What could be the reason? Could i be having too much of hindmilk (though it might be a stupid thought).I am asking this ques again as i didnt get many responses to relieve my tension.Mommies,pls tell me ur opinion.Thanks