How to reward my dog for desensitizing?

I have a 7 year old dog. He has been a country dog for the past 5-6 years, and he really never saw or interacted with other humans or animals. He is best friends with my mom’s dogs and we do have cats he is fine with, but there’s never really been situations where he is in close proximity with humans/dogs he doesn’t know.

We just moved to the city a few months ago, and we have a fenced in yard to play fetch and things, but we live across the street from the park that we go to walk at. He’s definitely reactive to people and other dogs since he’s not used to it. I know the best way to desensitize him is to just keep going, but I’m struggling on when to reward him during our walks.

With people he is off/on. For the most part at this point he’s fine seeing humans. We do have the occasional instance where he will growl. He is worse when seeing other dogs. He knows the command “look” to look at me, and he will do it when he sees another dog or human, so I think that’s good since he’s not 100% locked in on whatever it is. Usually when we see another dog I put him in a heel position and have a treat in my hand and continue to tell him to look at me as we pass them. Put there is instances still where he tries to pull towards the dog and growls. And even when he is in the heel, the hair on his back is still standing up and you can tell he’s still stressed. So I just don’t know when to reward him? I don’t want to reward him when he’s on alert and he thinks that’s what I’m rewarding him for? But I don’t want to wait until he’s chilled out and then he thinks he’s just getting a treat.

Thanks for any help/advice!