Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your wee one’s situation, and wanted to help.
I am very sorry to hear about Sonny's situation.
Furthermore, I am very sorry (but wanted you to know) that there is no OTC tablet chemotherapeutic you could use here for him. Cyclophosphamide,
the one chemo that comes in a tablet form, would be a very bad idea at the moment. The reason is because it could cause him to develop a hemorrhagic (bloody) ulcerative cystitis on top of his UTI (which I am sure you can appreciate would be a horrible complication for a dog with an active bladder infection).
In this case, I would advise that you need to focus on the infection. Ideally, we'd want a bladder culture to tell you what bacteria is present, so that you can choose the antibiotic that will best clear it for him. If you can clear the infection and get him back on track, then resuming chemotherapy may be an option. It will of course depend on his state at that point, but if he was doing well prior to the infection, then there is a chance that once it is fully cleared that he will be in a chemo treatable state at that point. So, it is a case of addressing first things first, and getting the infection away so that chemo can be potentially be resumed.
Overall, I am very sorry that there is no OTC chemo that you can use and doing so would potentially do him more harm then good at this moment of time. Instead, you need to focus on stamping out that UTI infection. Once it is cleared and especially if he feels better after it is cleared, he will potentially again be a candidate for further chemo.