Possibly from a habit of lying with the head to that side? I think it usually happens to the back of the head due to the recommendations for sleeping them on their backs. Not sure if/what problems it could cause. I slept DD on her side but always alternated so didn't have that problem.
Maybe try to notice if he tends to lie that way and encourage the other side by putting objects/toys/people there.
Sorry can't be of more help.