I have frequently prepared meat with no rub at all. I also try to limit both my salt and sugar intake.
A barbecued or smoked piece of quality meat requires nothing to enhance flavor. A good piece of meat taste good on its own
A good sear for the Maillard reaction is all that is necessary.
I have advocated this position for along time. I think you taste the meat more. I have only recently really started to explore different rubs.
But if you are used to rubs, you may feel that a rubbed piece of meat represents what you expect in flavor.
And frankly, the more flavor enhancers you use, such as salt and butter, the more you need.
I found that after cutting much of the salt out of my diet that I only need a few grains to enhance flavor and get the same effect as a few vigorous shakes used to provide.
And I cannot eat anything with butter any longer. It is so rich that it overwhelms my taste buds.
As an example, eat some lettuce greens without dressing, or a potato with nothing on it. It is amazing the flavors that we mask when we prepare food.
This is a minimalist view. You can add normal spices that you find in rubs, just minus the salt.
Others have already mentioned the usual suspects.
And keep in mind, if he is used to no salt then his taste buds will be calibrated to no salt.