I just wanted to share our experience with being off-the-charts small...
My ds2 weighed 6lb 10oz when he was born, compared to ds1 weighing 6lb 6oz.
At 1 year, ds2 weighed 17 lb, compared to ds1 weighing just over 20 lb. At least ds1 was on the charts, at about 25% I think. But ds2 was out of range, as you must well know!
So I worried some, mostly because ds2 had always been a "puker", spitting up if I nursed him more than about 5 min at a sitting. Even when he started solids, if he ate anything too quickly it would come right back up. I got to where I could tell from the expression on his face when it was about to happen so I could move his bowl out of the way!!
Anyway, as it turns out I was just about exactly the same size as ds2 (tiny!) when I was that age. He is now 4.5 and weighs around 30 lb. But he is sooo healthy, smart and strong. He is just small. This is what is normal for him.
My advice to you would be to keep track to make sure he is still gaining steadily (even if slowly) and is healthy. Also find out what you or his father weighed at that age. Maybe it's just normal for him!
I can't imagine formula would do anything better for him than your milk does. Even if he's not eating a lot of solids at a year, that's not a huge deal. I've known children who, at a year, wouldn't let anything in their mouths except for their mother's "nursies" and they were all healthy!
Good luck to you. I know it's scary to go against the doctor's advice, but you know best...