I sifted through a lot of pictures for you. I say darter.
Juvenile Round Goby linked by Schiz:
ID Chart for round Goby:
Another:
Yours doesn't seem to have the same finnage and spot on the dorsal fin.
Various Darter (Etheostoma sp.) Pics:
Hard to say what exact species of darter it is. There are a lot that look similar. Swamp Darter is a good generic term for it.
They are really interesting in the aquarium; lots of personality.
One thing is for sure, it's not a mudskipper. Prove it to your dad by putting it on some mud and watch it flop instead of skip.