Percentiles should stay rather consistent once established. As long as they follow their curved (stay around the same percentile) they are fine. Some babies are smaller, just like some people are smaller. My first was born fairly normal, but lost weight right away and didn't regain it. He followed his curve and stayed in low percentiles. At 3, he's a bit higher, but still super skinny. My 9mo old's upper arm is bigger than my 3yr olds. It bizarre. But all kids are different. Just like adult bodies come in different shapes and sizes, so do kids. Don't stress about it.
My doctor doesn’t care to look at percentile she looks at how healthy the baby is, my son was 6 pounds 14oz at birth and was past full term and was only 20pounds at his 12 month check up but the doctors showed no concern because even tho he wasn’t gaining weight super fast he was still gaining weight. He is in a low percentile but all babies are different and as long as you’re doing everything you can to help him grown don’t bet yourself up about it. Also did you doctor tell you that’s his percentile or did you look it up online? Because percentile growth is different for every child it’s not 1 size fits all.