4 week old - side sleeping

knottie6650924a9cb92839

My baby just hit one month and is now sleeping on his side. I keep gently rolling him back and he continues to find his way back to his side. Is it because I swaddle him? Or that the mattress is super firm and uncomfortable? 

How do I correct this? He sleeps in the newton breathable bassinet. 

knottie27a3ea4856d24aeb

I often would find my newborn shift to his side because of his swaddle. I recently switched to the weighted swaddle from Dreamland baby and it’s helped him sleep on his back without him moving to his side so much.