I think the first step is to get your son to actually go asleep in his own room. This is an important step for sleep association. " Easier said than done!" I hear you cry. I had this same problem, so I made absolutely, completely sure that by bedtime he was totally and utterly knackered, without the will to get out of the bed once he'd settled in! This may mean that you have to keep him up later for a time but don't let him fall asleep in his brother's room. You will have to be vigilant and it will be hard work but if you conquer this stage, then the rest will be comparitively easy! If he does get up, despite being so tired, he must be returned to bed each time. No talking, no annoyed signals from you, just a calm taking back to bed. He may be like a jack-in-a-box but you must persevere. At first, we spent 2 hours doing this and then it got less and less.
One year on he is staying in bed, sleeping through (yee haa!) and not getting up ridiculously early! He is now 3 1/2. Good luck and don't buckle or cave in...think of the bigger picture!