reminds me of when i helped a mate sort out his plumbing company's ads - he was about to throw money at Facebook first. Glad you've stepped in for your brother, HVAC is a proper goldmine if you don't mess up the setup.
start with Google Local Services Ads, not the usual search ads. You only pay per lead, not per click, and your brother only shows up for people hammering "hvac near me" or "ac repair [city]". He'll need to get licensed and pass a background check, but it's worth it.
next, lock down his Google Business Profile. Fill every field - hours, services, photos. Then hound every happy customer for a review. That's free and outperforms most paid stuff.
For Facebook, target homeowners in specific postcodes, aged 30-65, with interests like home improvement or real estate. Keep the ad simple: "Same day AC repair. Local family owned." Use a photo of your brother leaning on his truck - not a stock image.
Stick to this order: Local Services, Google Business Profile, then Facebook. Don't blow a penny on anything else until those are humming. good luck to him.