First of all- please don't panic! 
It's very common, and most round out nicely without any special treatment.
Please do check with your GP that baby doesn't have a neck problem (called torticollis) that stops him from turning his head to each side evenly. If he does have this, your gp will give you a series of physio exercises to do each nappy change with him, and it will get better pretty quickly.
Otherwise- keep him off his head as much as possible- in a sling, your arms, in a bumbo when he's a bit older, tummy time when you can (though my 2 hated this with a passion). Try and get him to sleep on a slightly different part of his head each sleep by using a rolled up towel under his matress- alternate the shoulder it's under to make him move his head left one nap, right the next IYSWIM.
We also used a goigoi pillow for nappy changes and when he was under the baby gym (only 10 mins max really, never unattended) - but many people don't like them as NHS staff say not to use, but in Sweden all babies have pillows pretty much from birth.
See how it goes- my son's righted itself within 5 weeks of doing these things (no torticollis, a very calm, still sleeper)