It sounds as if you have an active one on your hands, but it is not unusual for two-year olds to be interested in climbing. I will answer your specific question and highlight a few other issues you may want to address as well.
Regarding the crib, the first and admittedly obvious step is to make sure the crib mattress is in the lowest position. I encourage parents to lower the mattress well before climbing tendencies are exhibited. Secondly, remove anything that would help him climb out of the crib such as toys, stuffed animals or crib bumpers. Finally, if you really need to keep him in the crib, you may want to try a crib tent. This is a zippered mesh dome covering (available from Tots in Mind) that is intended to keep bugs out but also serves to keep toddlers in the crib.
Climbing over safety gates is a real problem as most gates are only 28 – 30 inches tall. However, Mommy’s Helper has recently introduced some well made extra tall gates that are roughly 36 inches tall. I anticipate that many manufacturers will be introducing taller gates in the near future.
If you have a climber on your hands, and it sounds like you do, you should be concerned with a few other issues as well.
Falls from windows can happen in an instant often resulting with tragic consequences. Make sure to remove climbable furniture from beneath windows and consider adding secondary latching or window gates. Any secondary latching should allow the window to open a few inches for ventilation but prevent a child from opening it further. Hardware to achieve this is available at most home centers depending upon the type of window involved. Whatever hardware you add, be sure that an adult can readily disengage it in the event of an emergency.
Climbers can also get into trouble if something they climb on were to topple over on them. This frequently happens with bookcases, bookshelves, changing tables and especially dressers. A child can pull open the drawers and then climb up them as if on a ladder. Unfortunately, the weight of the drawers and child combined to topple the dresser on top of the child. The solution for all of these types of furniture (and those prone to toppling in an earthquake) is to brace them to the wall. My favorite bracing hardware is made by McCaw and available at ACE Hardware.