Worst thing to do is google for answers it'll just scare you lol!! Its normal for babys to jump at loud noises just means his ears and reflexes are working well, I'm 28 weeks and my lg jumps at loud noises too! I was measuring very small at 36 weeks with my last pregnancy had a scan and she was fine was born at 40 weeks 7lb8oz x
babies start to really be aware of things that are going on, on the outside, they startle just as much when they're newborn for a few weeks until they get used to the noises :)
Why would you suspect Fetal Alcohol Syndrome? Is it a possibility? (Babies don't get that unless the mom has been drinking heavily through the pregnancy.) My baby used to jump whenever I coughed, he really didn't like it. No damage to the central nervous system.