In Freak the Mighty, why did Maxwell and Kevin choose that name?

In Chapter Seven, as Max and the Freak leave the grounds, they head for the lemonade stand nearby. Freak is still on Max's shoulders and spots Tony D. and his gang heading for them. Freak navigates Max away from the approaching punks by digging his left heel or right heel into Max's side, like directing a horse. They are able to escape the gang but Max is uncertain about the Freak's thinking when he heads them right for the pond. Max follows Freak's lead, however, and winds up stuck in the mud in the middle of the pond. Tony D. is short and when he tries to follow them, he is sputtering water. Saved by his gang, Tony D. starts flinging rocks at Max and Freak. The Freak whistles loudly as he sees a squad car approaching. He yells that they need help. The gang runs off and the police assure the boys that they will keep an eye out for the thugs. One of the cops recognizes Max as "Killer Kane's son." However, the Freak tells them that together they are to be known as Freak the Mighty.