I thought you didn't. Locks that were waxed once or twice may seem fine down the line because loose hair covers up the wax, but it is still inside them. It traps water and causes them to rot. It might not start smelling right away. But when they smell like a wet dog after he showers, even if the smell goes away when they dry, that's a sure sign that mildew or mold is growing. Many people think that if they smell fine dry then nothing is wrong. That's a myth. Mold doesn't always smell all the time. But it does eventually get a musky odor that is constant. Other may not smell it right away, but given time, as it grows more and more, others will smell it too.
Wax also forces locks to not mature and stay in their infant stages. It is just glued together. I'm sure you know. When waxed locks finally have the wax removed, people find that their locks are no where near as mature as they thought. This scares many people into ignoring the simple fact that something you'd use to waterproof your car does not belong in your hair.
I'd keep trying to explain to him that wax is damaging, but don't judge him on the fact that he wants to keep it there. When he finds out that wax is ruining his locks, let him come to the conclusion on his own and don't say "I told you so". This will make him reject your help later down the line if he wants to grow them again.
If you want to, you can use my story about what wax did to my hair. I think it's in the featured discussions under recovery. Not sure. It's definitely in the dreaducation page. Or message me for a detailed explanation