I put my dogs in the 40 or 60 when I'm in the garage.
I also take them places they love in the 40, so they associate "fun" with it. For example, I take them to go to the pond to swim or to a big open field to play ball. So that type of stuff, plus the treat training (treat him when he gets in the 40) will help a ton. He'll come around.
That is great it is going so well. Yeah, socialization is HUGE. Get him socialized with people and other dogs, but be choosy about the other dogs. Sometimes you get a bunch of idiot owners and dogs with zero training at dog parks.
From my experience some of the best dog socialization has been at dog training classes in a group setting as well as off leash trails. Most of the off leash trails here are not like an off leash dog park.
You have to have a dog that listens and obeys at an off leash trail. Obviously your pup isn't ready for an off leash trail yet, but just throwing that out there for future reference...in the even you even have off leash trails there.