Whatever you use, start it quickly. Because he's very sick, you haven't got a lot of time to lose. True, antibiotics won't help if it's a virus, but I've always been told by vets that they won't hurt either as they can help prevent secondary infections. You'll know if it's a virus if he doesn't start improving within a few days.
I personally think that using injectible Tylan three times a day is extremely stressful, not only for the bird, but the caregiver also. And I would think it must hurt if the bird has lost weight and doesn't have much breast muscle. I always think the less stress the better is the key to helping birds recover.
Tylan orally, yes it's good. I have never used Tylan, although I'm not opposed to using it.