Feline Urinary Infection?

My outside 8-year-old neutered male cat developed a UTI after a couple of antibiotic shots for another issue. He was prescribed 2 weeks' of Clavamox, but after a week, I had to skip a day because he had a hard time eating because his stomach was upset. (He's always had a sensitive stomach and usually vomits after moist food or even plain chicken. Has to have dry food.) I stopped the antibiotic for a day because he vomited and hardly ate, put a 30c pellet of staphy and a 30c pellet of cantharis on his food for a day (of course with dry food, he's not getting it all) and a couple of licks of plain yogurt for good bacteria, but then he excessively licked his penis after urinating, so I had to gave him another dose of antibiotics. We're leaving for the weekend (Fri-Mon), and a sitter will feed him and my other rescues, but I'm really afraid something's going to happen to him. He's skittish about putting anything in his mouth and smells everything on his food. Any suggestions? A day without the antibiotic, and he started eating more. Thanks.