In Multiple sclerosis
,apart from muscle symptoms,there will be bowel and bladder disturbances.
You didn't mention the reason for taking low dose antibiotics for several years.
If he develops UTI,he can take a course of Anitbiotics and stop once the symptoms are relieved.
Routinely,Urine will be sent for Culture and Sensitivity.E.Coli
is one of the commonest organism isolated in UTI.There may be few drugs which will work on the particular organism isolated and will termed as Sensitive.Some drugs will be have no action on the organism and will be termed resistant.It is common.
Please consult your Physician/Urologist
and take drugs as per their advice.
All the best