You've got it right by keeping it short. Young horses, well, horses in general, but especially young horses don't have great attention spans.
I'd lunge him rather than round pen him, then after you've got him going well on the lunge like try round penning him and in an area without grass if possible? He might focus on you a bit better if he's actually connected to you and can't wander about willy nilly. It's nice if they listen to you right off the bat, but if they don't, why put extra temptation in their way. You can use it as a test of your abilities once you do have him going well.
When lunging or round penning, walking or stopping is a reward. You may be being a bit soft on him, after he gets up and listens to you, by all means, let him walk and praise him, but until he does you need to be on. His. Butt.
But, my number one recommendation is that I would try to evaluate your body language. That is of great importance when doing either thing. When you ask him to move out, are you standing up tall? Are you driving him with your shoulder tilted and standing at his hip?