My current dog was trained and reliable on or off leash at 5 months of age, but at 6 months she started blowing recall anytime she decided she'd rather go or do something else! She'd look at me, but choose to go another way.
To teach her that recall (or any other command for that matter!) was not optional I left a leash on her indoors, then called her to a location in the house that she didn't want to go to (for her it was the upstairs bathroom, because she got baths up there and she didn't like being in the tub). I would call her twice, once in a normal voice, and if she didn't respond, again in a more commanding tone. If she still didn't come I'd calmly walk over and grab the leash, and without a word, literally drag her to where I was when I called her and then praise lavishly and release her. For her it took only 2 repititions of that for her to "get it", the 3rd time she came on her own....and continued to recall both on and off leash until just recently.
She still always comes, but recently when she's persuing something, another dog or a squirrel or rabbit, she's started to continue the pursuit and then come after she's met the dog or lost track of the small animal - which is clearly unacceptable! The point being, even when they "know" what you want they will test you from time to time!
As others have said, since your dog is so new to you, I'd just wait awhile and get to know him better before working on obedience with him. It takes at least 2 weeks for them to settle in, and sometimes depending on the dog 3 or 4 or more. Give him time to adjust first, then show him commands are never optional using a leash. In my experience once they've become reliable on leash with sufficient repititions, it usually transfers pretty to off leash as well, but again requires sufficient practice.
It's not the size of the dog in the fight, it's the size of the fight in the dog. Mark Twain