Coming from a Nathan, the three invariable ones have already been covered, IE - Nat, Nath, or Nate. But I wouldn’t call it being razzed, I’m perfectly fine with any of them.
A bullfrog? That’s an odd name. I would have called them chazwazzars! 
When I was a kid in New Zealand, I had a friend named Gary, called Gaz, and another named Barry, called Baz, and later a friend named Stephen, called Dippa.
Most of my other friends had regular names.
As in Anthony Buckeridge’s tales of life in a British posh school for pre-teen boys: there’s a lad in those, whose name is Charles Adrian Temple. His peers call him “Bod”. This is because his initials spell “Cat” – so the kids started calling him, instead, “Dog”; which in due course was “shortened” to “Dogsbody”, and then genuinely shortened to “Bod”.
I had to answer to Sparks for a while because my initials were the same as the then electricity supplier and my generally depressed state lead me to be about as exciting as shower mould.
A friend of mine managed to acquire the nickname “Othy” first year of uni because he went around saying “My name is Timothy, don’t call me Tim”.
I remember that former world chess champ Garry Kasparov was “Gazza” to his near-peers. Whether it started in Australia, I don’t know.
Don’t forget dutchy if his surname is Holland!
For Nathaniel he would normally get Nat or Nate or maybe even Nel!
I work with a lot of African Americans who go out of their way to name their children after African people or culture. Which I think is great, what gets me is they almost always shorten it to something like Kiki or DeeDee and I’m like, then what is the point to go to the trouble of naming your kid something with meaning to you, if you never use it.