Thanks to everyone for your advice. Wow, I had no idea about how bad soy milk can be -- I will definitely bring that up with our pediatrician, since she recommended it.
I tried making a smoothie with the soy milk and his two favorite things: banana and cinnamon. Still no luck.
We'll try the rice milk. Hopefully that will do the trick!
Incidentally, my husband and I are both average height and weight, but we were both above average on the growth charts as babies and toddlers. I am not really concerned that he is a shortie, especially since we are not tall -- I'm more concerned that he only gained three ounces since his 9 month check up (although he did grow 1.3 inches).
Thank you again!!!!!!!