The elder McClay, who was at the scene Friday morning hours after the shooting, said his son had expressed concern about his safety.
Police were looking for the pair who entered the store about 10 minutes before closing.
Officers found the manager shot in the head, police said, and he was pronounced dead at the scene.
Jason McClay played football at Haverford and studied criminal justice at Delaware County Community College, his father said.
He had served on the supercarrier Kitty Hawk, Bruce McClay said, a member of one of the last crews before the ship was retired. His son received an honorable discharge after being injured, he said.
He said the victim was the oldest son of a large extended family, with four brothers and one sister.
Jason McClay was single and lived with his sister in Broomall. He had worked at other Rite Aid stores, including in Exton, his father said.
"He was a good young man," he said.
Marjorie Reily, the victim's mother, is devastated, Bruce McClay said. She was in Ocean City when she heard about the shooting, and her husband, Jim Reily, went to the scene, Bruce McClay said.
According to his father, Jason McClay had said he was concerned about the lack of a security guard at the store. "It's dangerous down here," Bruce McClay said. "He knew the risks. He wasn't naive."
He said it was a shame Rite Aid did not have a guard on the premises, adding, "It's all about the money."
Rite Aid declined all comment beyond issuing a statement in which it announced a $10,000 reward for information leading to an arrest in the slaying. Later, the company said Purdue Pharma L.P. was chipping in another $10,000.
Police released surveillance photos of two people described as persons of interest.
The woman was described as white or light-skinned, in her mid- to late 20s, wearing a red hooded jacket with white stripes down the sleeves, blue jeans, and gray sneakers.
The man was black, in his late 20s to mid-30s, wearing a white T-shirt over a black long-sleeve top, rolled up blue jeans, and black boots.
The store was closed Friday morning.
In announcing the reward, David Mahan, Rite Aid regional vice president, said, "Today we are grieving the loss of a member of our Rite Aid family."
He said employees would be offered grief counseling. "It's a tough day for Rite Aid," he said.
Chester police asked anyone with information to contact Chester Police Detective Nelson Collins at 610-447-8431 or Delaware County CID Detective Dave Tyler at 610-891-4708.
Contact Mari A. Schaefer at 610-313-8111, firstname.lastname@example.org, or follow @MariSchaefer on Twitter.