Is there a karmic origin of his physical symptoms (strep infections, etc.)?

Yes, there is a karmic origin. As you have been shown, this is a reawakening and resonant sympathy with past lifetimes of chronic infection. So this is the scene having been set by karma to increase the vulnerability to microbial infection and then to see this inadequately handled by the immune system and become a chronic problem. This is the karmic potential for having had prior lifetimes with this experience. And this is not a sentence or a mandate, but left unattended will often re-capitulate, and the current lifetime will have a similar, or in some cases a more dire outcome from the repetition of the karmic episode of the past.