My boyfriend is losing his hair. What can he do to stop the inevitable?

Depends. There are treatments available for hair loss but you have to stay on them and they may have sexual side effects like Propecia (finasteride) does. Also Minoxidil is available otc . Again these may or may not help much but hair transplantation is an option also-- although a very expensive one! see if your boyfriend is ok with losing the hair or not. Then i would discuss with him the priority he puts on this.