I have 4 children. they are 8,7,6,and 4. When my 4 year old was born, my oldest was only 3 1/2. All my little ones got up out of bed and "explored". They are exploring their limits. It was fruturating at first. I had to go upstairs, put them back in bed, and then come back downstairs. I got grumpy about it at times, but punishing them never worked. You just have to be patient. He will eventually stop. He is just liking the idea of getting out of bed. It took mine a little while, but then I was right back at it with the next child. You learn to love the sound of their footsteps on the floor. That is something that I now miss. It was like music when my husband and I were downstairs. We would hear walking, then we would tell them to get back in bed, which eventually you would be able to do. Instead of going to his room. Anyway, we would tell them to get back in bed, then we were hear running footsteps, instead of walking. It will happen. Just be patient. Learn to listen to his footsteps with a smile, and it won't be so hard. Just don't let him know that you are smiling on it. It is kind of like when we mess up with God. He always picks us back up, reminds us that what we are doing is wrong, and then lets us fall again. We have to be patient, the same way God is patient with us. Good luck.
T.