J.P.
V.,
All of my children were moved to a "big boy/girl bed" by two years. They were also climbing out of their beds.... I can give you a few tips that worked well for my children, and worked well so far with my grandchildren.
1. Buy a bed rail. I got ours at Wal-Mart. I don't know if they still stock them. You can probably even find them at Babies R Us or some other place. The bed rail is usually about the length of the bed, and gives a sense of comfort while protecting your child from falling out.
2. DO NOT lay down with him! He will become used to that and you will have the dickens of a time breaking this habit. Your son needs to go to sleep by himself.
3. Be prepared for a few rough days. This is a new transition for him, and he will need to become comfortable again with the bed. Be patient and know that this, too, shall pass. He will become a champ at sleeping in his own bed.
Best of luck!