It is difficult when our children do not sleep through the night in their own beds. When children learn sleep patterns, they learn not only how to go to bed (which your son seems to have no trouble doing) but also getting back to sleep when woken. It appears he does not have this ability. He has developed a pattern of only returning to sleep when he is in your bed. So, you need to retrain his brain.
Dr. Patrick Friman has written an excellent and somewhat funny book titled
Good Night, Sweet Dreams, I Love You: Now Get into Bed and Go to Sleep! You can find it at
boystownpress.org. The book talks about continuing to be consistent and calm and offers techniques that you can try to help your son learn to sleep through the night in his own room.
One method that may work for you is the robotic return. Hold his hand and calmly walk him back to his own bed without making any eye contact. Be like a robot, you are not upset or cuddly, just returning him to bed. However you decide to approach this issue, remember to stay calm and focus on praising his positive change.