There are many beaches that would work where you can get away from the crowd that are closer than Le Galion.
If it were me, I'd use Cupecoy. Not many people usually. Beautiful but one of the smaller beachs. it is a clothing optional beach so he may not want to do it there.
Since you are going in January, it will be more effort to get alone anywhere you go.
Or even better if they swim/snorkel. Snorkel out of Baie Rouge along the right side rocks. You can swim through a stone arch near the headland. On the other side is a small beach that is only accessible by swimming there.