Baby Hates Sleeping Flat

My baby is 7 weeks old. We had him in a bassinet and he was not sleeping well at all. He was sleeping better in the napper part of the pack n play, so we put him in that to sleep at night as well. However, he's now way too big for it and still hates sleeping on his back in his bassinet. I've put books underneath his bassinet mattress to help elevate him, but he still hates it. He could be passed out but the second I lay him flat in the bassinet or even the flat part of the pack n play he's awake and screaming. I don't know what to do and feeling nervous about going much longer like this because I don't want him to not sleep, especially when I go back to work in two weeks. Any advice or experiences with this?