I had total luck with the Pampers (the Swaddlers one (Crusisers when she gets older), not Baby Dry) brand. For night time, I placed the 'next size up' on my little one. So if she wore a 4 during the day, I used a 5 at night. It may seem a little big on the torso, but all that space is great! For us it worked really well. Oh yeah, for your son remember to point down. :-)
As far as the tummy sleeping, once they start learning to rollover as mine did at 3-months, all the back sleeping will be out the window and I think they naturally prefer to be on their tummies. We asked our pediatrician back then at her 3-month check-up and was told that once she could roll by herself, there is not a whole lot we can do to prevent it. Make sure to talk to your pediatrician about it at your 3-month check-up.
Check out this link on the same diaper question. Lots of good info and the Mom replied/updated stating what worked.
www.mamapedia.com/questions/7425068348460761089