English is not my first language so forgive some errors.

His body mostly likely doesn't have anything to do with it. What we call "resistence" , at least for antibiotics, is purely linked to the pathogen.

When someone uses antibiotics with no advice from the doctors or pharmacists on how much, how to and when to take it, the worst case scenario is this pathogen developing some kind of mechanism that can preserve it's life when the same medicine is being used. The most common case can be observed with amoxicilin: if someone has already used it to treat some kind of infection in the past but didn't follow strictly the doctors orders, it's very possible that on a future infection with the same kind of bacteria it will have a survival mechanism strong enough to keep it alive against the same amoxicilin. In this case, the solution is to switch to another type of antibiotics, or a stronger one from the same family.

The kind of "resistence" that we can obeserve in humans is, in most cases, with medications that mimicry some neurotrasmitters. In this case, the "resistence" is basically your body becoming less sensible for one or more of those molecules (i.e dopamine). Example: You are using too much of some medication, so your body increases the number of receptors in your cells. By doing so, you need to increase even more the medication to "fully occupy" the same receptors and get the same feeling as before. There are other cases that can seem like "resistence" from the patient, like what we call "ultrarapid metabolizers": their enzyme, naturally, just process some kind of medicines faster and, because of it, they need to use more medicine to have the same effect.