I did the gerber growth tracker....it says my son is severly underweight?

Most of those growth charts, etc... are based on the average child. I don't think any of my kids were that size at 7 months, lol. Every baby is different, and not every baby will be a certain weight or height at a certain age. If you're really concerned, jot it down and take the concern to your baby's next check up. Some kids are just out right smaller (in both weight or height) then the "average" baby. So long as your baby is otherwise healthy, I wouldn't worry too much about it. As I said, ask your baby's doctor about it, they'll be able to give you better advice.