I am sorry to hear about Rubio's fishy smelling urine, and more frequent urination.
I understand that he has a history of weakness in his rear legs and trouble walking.
It sounds like he has a urinary tract infection. Those are rare in male dogs because they have such a long urethra, which usually successfully keeps bacteria out of the urinary tract.
In his case however it is possible that his weakness in his rear legs indicates poor nerve function in his spinal cord.
That can also affect his urethra's ability to function properly and keep bacteria out, and it can affect his ability to fully empty his bladder normally. When dogs cannot fully empty their bladder that predisposes to bacteria growth in the bladder.
I highly recommend Rubio see his veterinarian for a physical examination and urinalysis with culture.
In the meantime push fluids. The more his urinary tract is flushed out the better he will feel and more bacteria will be flushed out of his urinary tract.
Please let me know if you have any further questions.