I have heard of a few people recently with nasty throat infections..
Chicken bone broth, even if he only has a few spoonfuls. The extra ingredients will make it taste good so do add onions, garlic, carrot, celery, herbs, ginger, nigella seeds, cardamom, salt.
Turmeric 'milk'.. made with rice milk, turmeric, honey, black pepper and other spices like cinnamon and cardamom.
Vitamin C, vitamin D, zinc. Look for liposomal liquid drops which will be easier to take than tablets. (Amazon)
I would also look at probiotics. It sounds like he has really been knocked by this but also has underlying issues so I would keep things simple, rice, chicken soup, steamed vegetables. Building him up gradually and allow his body to heal.