Tylan can cause some muscle damage in some cases, so that is why most chicken owners give it orally. As long as the crop is emptying normally, it should work as well. The injectable will get to work more quickly when starting out.
Since he is improving, if you are giving Tylan, just give it orally. Dosage of Tylan 50 is 1/4 ml per pound of weight given orally 3 times a day about 8 hrs apart. Let us know if you have something different.