I understand your constraints and I appreciate all of your rescue efforts. While of necessarily questionable value, a broad spectrum antibiotic such as tylosin is reasonable. This is given by injection, however, in the form of Tylan 50 dosed at 20-30 mg/lb once daily for 5-7 days into a breast muscle. If stressing him in this manner needs to be avoided, you're left with treating through his water with tetracycline in the form of Duramycin-10 as per the label's instructions.
In general, I'll offer such an ill but appetent bird anything and everything he'll eat. Our goal is to increase his protein intake but, frankly, increasing his fat, vitamin, essential fatty acid, and mineral intake is desirable as well. Turkeys are omnivores and can eat hard boiled egg yolk, pancakes and cornbread, the tops of fresh greens, dairy products such as yogurt and cheese, fresh fruits such as apples, pears, melon, kiwi, and berries, vegetables such as broccoli, carrots, beets, asparagus, cabbage, sweet potato, and squash, and even tiny pieces of meat.
Please continue our conversation if you wish.