"...my OH gets frequent UTIs. Are they related to personal hygiene?
They could be but it is less likely in a man.
Dementia can weaken the immune system as can antibiotics.
The best thing to do is to keep up the flow so keep offering fluids, as has been said an enlarged prostate doesn't help.
Cranberry juice may help, it can't do any harm unless he is on warfarin in which case he should not take it.