How can I help my dad bathe at least once a month?

Use the search function, with parameters on "bathing",  "no rinse products", "safe bathing" "unwilling to bath", or similar topics.

Bathing, whether to use immersion bathing in a tub, benches, and a lot of issues have been addressed here over the years.   You'll find you're in good company; this is a frequently asked question.

For my Mom, the problems was the room was too cold...and too cold getting dressed, etc.

so, I would start by running a space heater full tilt in the bath...putting all the towels on high in the dryer. She was much less reluctant once she was kept warm. Then drop her clothes into the dryer and grab the towels. Nice hot towels to dry with...really a comfort. Nice warm clothes to put on.

anyway, figure out what the issue is and work on solving the problem he has. Maybe too cold? Maybe doesn’t want to risk falling?