The only thing I would add to Kira's explanation about marker training:
Timing is important. No more than three-seconds between mark and reward. As he gets older, you can wait the whole three-seconds...and as he gets even older, you can wait up to six-seconds, but no one suggests going over six-seconds.
I'd stay away from using "good boy" as a mark. First, we say "good boy" without thinking when we're not training, so it gets diluted as well as confusing. I use "Yes!" Quick, calm but upbeat...different than you'd otherwise say the word.
Treats
>>Use treats that won't crumble. You want him to focus on you, not on the floor trying to gobble up crumbs.
>>Switch up how the treats are delivered so he isn't always looking at your right hand after you mark. Use both hands, hands from behind the back, etc. Probably not for a puppy, but I shoot treats (like a small, solid chunk of chicken) from my mouth, too. (It's pretty funny the first time; they sure weren't expecting it. Good for learning "watch." And they learn to catch reeeeeaaaaalllll fast. Just take care about the distance between your face and the dog's until he gets the hang of it. In the event he gets a bit rambunctious and anticipatory and jumps up, don't treat. Start again.)
Just my two
for now.
Pups cannot just shift from super play to stop, they need to wind down. I would also highly recommend a collar and leash on him at all times, so when he gets into trouble, you can get to him. 
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