3 month old sleeping too late! help! why the change?

It's 2:13am and my 15.6 week old is wide awake. He refuses to sleep. He was fed and changed. All he wants to do is hang out and play. I tried many time to swaddle him and put him to sleep but nothing is working. This has been happening for the past 2 weeks. He used to go to sleep at 11:30pm and wake up at 6:30am. Please help! Why is this happening and has this happen to anyone else?