I'm sorry you're having this problem. What form of cephalexin do you have?
Dop you have any other antibiotic?
How badly infected are his wounds? Is there discharge. . . Is there an abscess?
I would give him 10 mg per pound 2 times daily. You will need to mix it with baby food and syringe feed it to him. Tuna juice will work also.
Make sure he is eating for you as well. The weght loss you describe is not normal for a cat with infected wounds. It makes me think there is another issue going on. He should have a viral test for FELV/FIV.
I hope this was helpful. Please feel free to follow up with any other questions by pressing reply. Thank you for your patience. The site had some sluggishness today.