The beating occurred at about 3:30 p.m. Friday at the Sports Plus 1 store on Mission Boulevard, Alameda County sheriff's Sgt. J.D. Nelson said.
The victim is in critical condition at Eden Medical Center after being beaten with what appeared to be a baseball bat, Nelson said.
An investigation into the beating led to the arrest of suspect Aaron Tomlin on Saturday, and sheriff's deputies are still searching for a second suspect named Christopher Echols, Nelson said.