As an adult dog, your dog falls out of the several month window that dogs have as puppies where they learn behaviors that are imprinted on them and that they tend to take into their adult life.
Does that mean they can never be socialized with other dogs or people? Absolutely not. It is important not to rush the situation. You just got your dog and July 4th is around the corner. What better way to introduce him to friends and family than by bringing him to the fireworks show? That’s like taking someone that doesn’t know how to drive that putting them on the freeway at 70mph. It’s overwhelming, potentially dangerous and non-productive. If anything, that might take your dog multiple steps backwards as they will view the situation negatively.
There are many classes offered in socialization at the local pet shops, but if this is not your alley, you can always introduce your dog to other dogs and people, but we would recommend that you do so on a person by person basis. Remember that dogs react to your behavior, so keep leashes slack and your demeanor calm.
If you want to see how your dog reacts at a dog park, maybe you should taken them outside the gates and see how he reacts to all the unfamiliarity. If other dogs approach, give your dog a small treat and associate meeting new people in a positive light.
Dogs are not big fans of change. They like routine and can live perfectly full lives even if they never end up at a dog park. However you choose to introduce your dog to new members of the family and their pets, make sure that you take baby steps and if needed consult professional help.