Hunter looks like such a sweetie! He's a young boy and so is your relationship with him, of course you will bond and become very close, but you'll have to do a little training first so he has a fairly reliable response to your come command, before letting him loose in the park. He wants to please you, but you need to show him how, and what to do.
With all my dogs, I trained them with a retractable leash, I have 2 old Flexi brand 26 feet that are still in perfect working condition after decades of use. First you have to get some tiny, really tasty reward treats. They sell small freeze dried liver and other flavors for dogs that are small. The good ones are expensive, but you'll only be using them for training, and not giving them freely as treats.
If you decide to try this way of training, use the retractable leash in an open park area where there are no dogs around, a regular park. Walk him casually and let him go the full length of the leash, have one treat ready at hand in your pocket.
The in a friendly, upbeat and positive way, say "Hunter, Come!" If he comes to you right away, give him the treat reward immediately and tell him 'good come', 'good boy'....with a short pet. Then keep walking until that lesson is forgotten and repeat. Do this several times during the walk, and don't ever give the treat until his recall is immediate and reliable. If you have to say it twice, it's okay, but no more than that for a response.
It's good to call him in a calm but happy way, you can also practice this in your yard. After a week or so, when Hunter has shown good result with coming to you on the retractable leash, try it without the leash in your yard. Go out with him and don't let him know you have reward treats in your pocket or there's anything out of the ordinary. When he's far away from you, or distracted by something, say "Hunter, Come!", if he comes right away, reward him immediately and calmly praise him.
Once he's coming reliably on the leash at other parks, and in your yard off the leash, try it at the dog park. I'm sure there are some dogs at the dog park that are still kept on lead, I know there are at my dog park. Keep him on the retractable leash, preferable in an area that's not too congested with dogs and people....off to the side. Have the treats ready at hand in your pocket, let him go far on the leash, then call "Hunter, Come!", if he comes to you right away, give him the treat and praise immediately and tell him 'good come, good boy'.
Don't give him those treats at any time, except for training. Once he's coming to you, you can just give him praise and no treats. Only then should you let him run free at the park, providing it is securely fenced in. When he's good enough to test off leash, keep the treats on hand, and test him frequently to come. When he comes, let him go play again.
This is only a suggestion that has worked for many years for me and my dogs, it's okay if you don't want to do it, I only want things to be good for you and Hunter and I want to help. Good luck to you and your new boy.
This is the newer version of the leash I have.
https://www.amazon.com/Flexi-Classic-Retractable-Leash-Small/dp/B00Z9OFWF0/ref=zg_bs_3024196011_5?_encoding=UTF8&psrefRID=AR5W5ERSBWW7476ACQAX
