I dont think it is a serpae. They have smaller eyes with longer more defined fins. There is also a longer finned version of the serpae.
Serpae Tetra (Hyphessobrycon eques) Profile
The Serpae and Black Widows are known for nipping because they have longer fins. Unless he is super baby and hasn't grown any of his fins out I don't think it is a Serpae.
Edit: in the 3rd picture it looks closer to a Serpae.
The last picture looks totally not like a serpae though....
Now i dont even know anymore...but I still think its not a serpae.