He's not going to 'build his retention' if he has an undiagnosed problem and dilute urine! Yes, old dogs can get 'soft' and bladder pressure not always felt to empty completely. My old boy has some mind issues now and I encourage him to empty by putting him where other dogs have peed, especially those girls.
Have you had a cystocentesis urine sample cultured for proper antibiotic if needed? One week amoxicillin is not going to help a persistant UTI. Neither is a free catch sample going to be accurate from a boy.
Check prostate, do regular bloodwork to monitor kidney and liver changes. Ultrasound?
May be early Cushings or 'Cushingoid' symptoms. You can ACTH stimulation test for adrenal function. Adrenal tumor?
Yes, there is behavioural water drinking, but usually starts because of physical problem in the first place!
I also had a young dog with allergy and inflammation who drank buckets and peed frequently because dilute.
Try a second opinion, be proactive and help him be more comfortable.
