Baby that constantly cries!!!!

Hello everyone. So I'm a Nanny and I've been a Nanny for years. I have 3 of my own Children as well. I Nanny a 4 year old Girl and a 5 Month Old Boy. The little boy constantly cries. His parents know it but aren't concerned. You feed him, change him, cuddle him and he will still end up crying within a few minutes. In order to get him to sleep you have to put him in the swing, put a vacuum sound next to his ear, and pat underneath the swing until he falls asleep. As I said I've had 3 of my own children (ages 6, 3 and 1) and I've never dealt with this. Could he be colic? Any other suggestions? Or tips on how to soothe him? I've had him for about 3 months and it's not getting any better. I almost quit yesterday, only thing stopping me is because I don't want them to find another Nanny because these kids don't have a constant in their life except for me as far as Adults. Please help! Any tips, opinions, thoughts?