I realise you said 'perhaps' but I have to comment on how much I dislike this kind of backwards reasoning. I used to compete in bodybuilding, and I heard arguments like this all the time. "I'm not making progress, I must have low testosterone", "I must not be metabolizing protein", etc. The number of possibilities is huge, you really can't say with any certainty what the cause is.

Most likely he just has a tolerance to caffeine from drinking it. It sounds like he drinks it all day.