Newborn under weight ?

My baby boy was born at 39 weeks on the 13th. He was an emergency c section due to a placenta abruption. Then they discovered that his umbilical cord was tied in a complete knot. He only weighed 6 pounds 7 ounces. At his first appointment 2 days after I got out of the hospital he dropped down 4 ounces. I noticed today when breastfeeding him I can feel his spine. Is that normal or is he underweight? He had his 2 week check up next week. But I’m worried he isn’t getting enough with just breastfeeding. Is it normal or something to be concerned about?