The suspect, Sammeon Christian Waller, 25, allegedly took his grandfather's minivan and fled to the Sacramento area after the alleged assault.
Police were called Friday afternoon to a residence in the 2900 block of Gayridge Street for a welfare check on an elderly man who had not been heard from in several days.
Officers heard a moaning sound inside and found a rear window smashed in. They discovered the victim, Robert McPherson, 82, on the ground bleeding and suffering from major head trauma.
McPherson was brought to a local hospital and listed in what police described as extremely critical condition.
Investigators learned that McPherson had an active restraining order out against his grandson, Sammeon Christian Waller, 25. Detectives consider Waller the suspect in this attempted homicide.
McPherson's vehicle, a 2003 silver Honda Odyssey was taken from the home and later found abandoned in Sacramento. Police say Waller was last seen in downtown Sacramento near 3rd and J streets.
He has no known means of transportation and no known ties to the Sacramento area so police believe he may be using public transportation.
Anyone with information is asked to call Pomona police at (909)620-2085 or provide an anonymous tip to Crime Stoppers at (800) 222-TIPS (8477).