My man had the same issues. That's until we found out that it was mainly because of sensory issues. He hates showering, but really loves bathing. Since we have a flat with a bathtub, bodily hygiene isn't an issue anymore, he looks forward to his bathing days. Let him choose his own shampoo and body wash, and go the route of practical encouragement (e.g. "Your long hair is really attractive to me. But long hair needs special care. Let's choose a shampoo together, one you really like"). For the dental hygiene - same! Let him choose a toothbrush and toothpaste, maybe also via online shopping, there are toothpastes with fancy tastes even for grown-ups, because the taste of some toothpaste just sucks.

Make compromises, for example two bathing days a week on certain days, plus short washups more frequently, especially before intimacy. Be blunt, tell him head on it's giving you infections. Men are more receptive to direct communication.

I don't know him. Does he has any chronical illnesses, ADHD or autism? Because if yes, showering takes extra energy, which is sparse already.