See with my first two pregnancies they were right on the nose with their weight but a few ounces.off. I mean it all depends on what measurements they can get of baby and how good your Dr and ultrasound tech is.
A friend of mine was told she wasnt gaining enough, they sent her for the same ultrasound, she was told baby was to small, a week later she gave birth to a 8.5 pound baby they said would be like 6-7 pounds.
As long as the baby is around 10lb or under you should be able to deliver vaginally. Also, you can decline that next ultrasound, you are NOT required to have it done. If you and baby are measuring fine so far and are healthy, there is no need for it. Just go with the mindset that baby will be a size you can handle and do a vaginal birth. If you feel "forced" to get the ultrasound and they're like "Oh, no. Baby is 9 1/2 lbs lets schedule a c-section" I WOULDN'T do it! This late in pregnancy using ultrasounds for sizing is very inaccurate and can be off by a couple of pounds either way.
Also, just because you are not gaining weight but baby is doesn't mean s/he will be big. Your metabolism slows down when pregnant so what you normally eat goes farther and our bodies extract more from it, so that's why baby is still getting the nutrients and weight gain it needs.
How big was he last time they measured him and how far were you? At 35 weeks exactly (last Friday) he was 6.8 lbs. But my other two were big boys as well. As long as you are having a healthy pregnancy and your body adjusts to the baby's size, like your hips spreading you should be able to try a natural birth.