Ive noticed my DS favours one side alot too...seems like its pretty common tho! We have our 6 week check tomorrow so I will mention it to the doctor! He will move his he to both sides during tummy time, but it seems a struggle to go to his "off" side, and that flat spot he has developed looking shocking lol poor little thing!
So sorry I have no advice other than what the other girls have said about mentioning it to your Dr/nurse next visit...maybe try putting a noisy toy on the off side too...