1) If your son really is scared then it would be evident in his reaction to the snakes. Most likely you'd have to drag him kicking and screaming over to them. If he's showing them off to his friends and wanting you to take the snakes to school, he's not scared.
2) Your ex wife does not get to ignore a court order and deny you visitation time with your son just because she is scared of snakes. Make it very clear that while you are sensitive to her concerns and your son will be supervised at all times, if she violates the order you do have the option of filing for contempt.