I hate snakes. I can not recall ever seeing one at WDW, although I know they are they are there. I live in the mid-west, no where near Florida and I have however seen one on my front porch. Over my lifetime, we have also occasionally had them in the yard, once in the garage that I can recall, and have seen them some other places. Unless you live in a place where the terrain is absolutely snake free, while your chances of running into them at WDW are not absent, they are probably not that much worse than elsewhere. They don't particularly care for people, so I think them being out in the open like the one at the gates at MK last month, is pretty rare. I would tell him to stay on the concrete paths and not put your hand in any bushes even if you have dropped something there. I actually have never seen one at WDW in the water- although we have seen them in the lakes at home-- but just if you are on a boat, don't put your hands in the water, and you can also stay away from the "beach" areas.
Now- as far as gators, we have seen those several times on WDW trips, including WDW property. There are just a lot of them in Florida. Probably 6 years ago, I specifically recall being on ME from the airport when the bus driver said "Wow- big gator everyone look left". There was a giant gator sitting right next to the Interstate. We have seen one on the WDW golf course from the bus. We did see one in the water by Splash Mountain- but not where you would be near him in a ride vehicle. We have seen a couple from a distance in the Bay when we were on a boat going to and from Wilderness Lodge. We have never once felt threatened by a gator we have actually seen. Again, tell him to stay off the concrete paths, don't put your hands in the water from any boat and stay away from the "beach" areas- or at least stay away from the edge of the water. Your chances of any type of gator run in are very low. We kind of enjoyed seeing them until the horrible accident at GF.
Not to try to scare him, but we did have one big scare. Based on that, I would add to my recommendations not to be anywhere near the beach areas at night if you already have a fear. This was 9 or 10 years ago at WL. We had gotten back from a park. WL had movie night at the beach going, and I took DD over to watch while DH went back to the room. It was dark. We watched for a little bit and then decided to go back to the room. The movie was not over, so we were the only ones walking back at the time. We were on the path, and something very loudly hissed at us. I would like to think it was not a gator, but it definitely was something, and I suspect it probably was a gator because I know that they hiss. I'm sure we probably startled him. We booked it down that path away from that hiss as fast as we could go. That is the only time, I have ever been critter scared at WDW... well unless you count the dinosaur ride.
I do think that with the horrible accident, they are probably being more pro-active in trying to spot and remove them, but it is Florida.