If children are growing at a steady rate in both weight and height, you do not have to worry as much. Some children are small in size. If the growth or weight starts to slow down, that is when Doctors start to get concerned. I do not suggest tofu or margarine at that age. I would continue him on his formula or breast milk and maybe increase his bottles. When you give his rice cereal, mix it with formula not water for extra nutrients. Mix some of the jarred baby meat into the jarred dinners. That will give him additional protein / calories. Do not give an excessive amount of juices and water during the day because that is empty calories. Hope this helps you.
I would not add margerine, have you read anything on how that stuff is made. 16lbs seems pretty small for 9.5 months, if your doctor is very worried, what has he suggested that you do? IF they have not given any suggestions, find another doctor.
If it was my child, I would probably give more fruits and veggies throughout the day, as a snack, along with their regular meals, and a bottle with each meal, 1 before they go to bed for the night. Just my opinion.