Do you have some sort of mobile, bell or other interesting item hanging above the changing table. Lots of distracting things for him to look at might help. I have a stuffed animal on a trapeze hanging over the table and just off to one side a set of bells that hang from the wall (I can easily reach and set them to ringing if needed, and it keeps the attention of a child for at least a few seconds).
I also have a small mirror mounted on the wall at the foot end of my changing table (which is against a wall in a corner). I let the child have a chance to look at himself in the mirror before or after being changed, and we talk about (and to) that cute baby in the mirror.
Another idea is that if the diaper is only wet, try changing him standing up if he stands well. If not, I have perfected changing a child sitting on my lap. Just put a towel on your lap, sit the child on that and go ahead and do the change. It may be awkward at first, but the child will likely be so surprised at this new position he'll cooperate better, and you'll soon be an expert at getting the diaper changed in that position .. and the new one will even be on straight!
Above all.. as others have mentiond.. keep up a conversation, singing or such with the child as you change him. Diaper time can actually be one of your best teaching moments as he hears your voice giving gentle commands, or just in conversation. Talk about what you are doing each step of the change. It's a good time to mention body parts... eyes ears nose, etc. or even 'bottom', since what you are doing involves his body.