Pixie is 5 months, and she is jumping, biting and pulling on the leash a LOT when we go on walks. More toward the beginning of the walk, less toward the end. I figure she's very excited about the walk and has a lot of energy to burn. I do stop and tell her 'down' every time. We have not started obedience training with a trainer yet; we'll be starting a class after the holidays and I can hardly wait, honestly. It's a lot of work to walk her. I use a slip lead and try to correct her every time I see her get that look in her eye, before she jumps, but I can't always catch it in time.
She does growl when she jumps, but it's her play growl. It still could be a dominance play, I'll be curious to see what the trainer says about it, but she has a mild temperament. I think it's just her telling me she wants to goof around.
Turning and walking the other way if they pull works really well. I don't know about the harness, I've never tried one (maybe I should!). It really sounds like you're doing everything right with Sam, and he just needs more time.