My LC said its very common for babies to prefer a side. She said its often chosen based on how they lay in utero. Just keep relatching. It only took a few weeks for dd to get over her preference.
This. My DS also preferred the right side. It actually took me 12 hours in the hospital to get him to nurse from my left breast at all.
Just keep forcing him to relatch and he'll get used to it with time. Also, if he's getting bottles (at daycare, etc), ask them to hold him as if he was nursing from the left side while he takes a bottle. That should help too.