Haas played with Head when he was younger, before Dunlop payed him to switch.
He switched back to Head at the US Clay Court championships, while still sponsored by Dunlop. About half way through RG, he made the switch to Head official. There's a thread detailing this somewhere on this site.
His racquet is a pro only mold (probably PT57E or PT57A) which is painted to represent the Microgel Prestige MP.