I was a late walker myself by a couple of months. And I never really did crawl. I can remember the fact that I just loved to roll, so I had no reason to do anything else. My mom told me that I actually ran before I walked hehe. So definitely don't worry overly much about that. As far as the charts go... the last chart I read was on healthy weight and it told me that according to my height/ weight I was obeise and a definite health risk. I found this hilarious since I only have 8% bodyfat. Charts are based an a national average and are not really all that accurate a predicter of anything. obviously, your son was way ahead in early stages but now he's at a slower stage. Before long he'll hit a faster stage again and be caught right back up and possibly ahead of things. I remember a boy in my class that was never over 5'6" all through high school, then i saw him a year later and he was 6 feet tall. Again, it's nothing to worry about.
As far as the MD goes... if your wife doesn't have it then it isn't an expressed gene in her at the least, the gene may not be present at all. If no one in your family has it then the chances of your son having it are very, very remote. Have faith and be confident, and I'm sure everything will be just fine
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"Life is what happens while you are busy making other plans." - Lennon