FRIENDLY advice please...toddler climbing out of crib?

My son is nearly 21 months old and very tall for his age. Just last night, we discovered that he can now climb out of his crib ALREADY! I really don't feel he is mature enough to be out of his crib yet, and i'm kind of at a loss as to how to keep him in it. I know they have enclosures that you can buy that go over top the crib, but i really want that to be the very last resort. He's not a zoo animal, ha-ha, but I will do anything for his safety. Another issue that compounds things is that I am 7 months pregnant with our daughter. She's due 6/30. I need to get a plan/routine in place ASAP before Tabitha arrives. Anyone that has gone through this transition from crib/bed (especially at this young age) as a mother or father, please chime in with your friendly, nonsarcastic adive. It would be sooooooooo very appreciated. Thank you very much! HELPPPPP!