I agree with the 10g idea. Plus, a 10g tank is actually cheaper than a normal 2.5g rectangular glass tank. It also opens up the aquascaping ideas, which he will enjoy.
I like the idea of dwarf puffers, I have one now. I have to warn you though, I have never seen them eat pellet/flake food. In fact, the two I have had in the past only ate small snails and guppy tails.
If you stick with the 2g, I say go with the endlers. If you go with a 10 though, (this is the most important part) you have more options, and you can take him FISH SHOPPING. I think we all know that visiting the store and grabbing an impulse buy is probably the most fun that anyone can have.
good luck, and good idea!