(It bothered him that his son's name sounded so typically Capitol - Marcellus).
He had planned on changing it when he first returned home and even recruited Haymitch to help come up with a new name. But Haymitch would not let him.
"The kid's too old for that. He can walk. He can talk. Leave his name alone."
And so he left his son's name be. But he did try to brainstorm a few nicknames.