My experience with flat coated Retrievers is that they are very energetic and very sociable. Since you've got a puppy, I'm not surprised you are seeing the extra-caffeinated version of those traits. I expect the problem is made slightly worse by the fact that he is in a new home and doesn't really understand the rules. He will need time and consistency to figure things out.
I'm glad to hear you are doing obedience work with him. Engaging his brain will help drain some of his energy. You have only been at it for a week, and he is a doggy teenager, so I expect almost nothing you have taught has become habit yet. Consistency on your part and maturity on his part will improve things.
What are you doing to expend his energy physically apart from the two walks each day? Remember, he is a hunting breed. His ancestors would have gone out into the woods and marshes to work for hours at a time. A walk on a leash expends very little energy compared to what he was bred to do. One reason so many dog owners get into the Agility sport is because they own dogs that need an outlet for their energy. A collie with no sheep, a terrier with no rats, and a retriever with no ducks can be a difficult animal to live with. Sports like Agility or Rally provide a better outlet than digging holes in the yard or chewing the woodwork. He's still too young for serious jumping or pole weaves, but he still needs some kind of cardio exercise.
I'm also not surprised he's not playing much when left alone in the yard. As I said, Flat Coated Retrievers are very sociable. As gun dogs, they were bred to work under the guidance and direction of their owner. They crave this companionship and attention. He probably thinks the yard is a boring and lonely place without you there. It's better than being bored and lonely in a kennel, but I think he really wants to be part of whatever you are doing. Of course, humans need to earn a paycheck; we can't just devote our day to entertaining the dog. However, it's still important to give sociable dogs as much attention as possible when we have the ability to. Right now as I type this my two dogs are lying half asleep near me, one on a chair and one on the floor where I will need to step over him to leave the room. The one on the chair is watching me with half closed eyes; he opens his eyes all the way whenever I move, and he relaxes again when I do nothing interesting.
As far as jumping on people and guests, does he do this when they first arrive and then settle down? Or does he keep pestering them for the entire visit? I have found it helps a lot to use access to the people as a reward. This requires the people to be on the same page as you: no patting a dog that jumps. When I get home my dogs are usually crazy. I say, "Hi Guys," drop my bag, and walk past them without giving them any other attention. If they are jumping on me I push them out into the yard and close the door. If they are simply spinning in happy circles I hang my coat, go to the bathroom, wash my hands, and do various other things --all without giving any attention to the crazy acting dog. Once the dog is behaving calmly I will pat him and give him attention. The important part here is that crazy dogs get no attention. Good dogs get to stay with the people.
