Hi! I don't normally post on here but I need some help!

My baby just turned 7 weeks old today and used to sleep around 4 hours in his bassinet at night and then every two hours after that. Since last week he has stopped sleeping in his bassinet more that a couple hours. He will sleep on me for HOURS but when I put him down he immediately starts fussing! How do I get him to start sleeping in his bassinet again? We have a set routine every night and it hasn't changed!