The accuser says Sandusky put soap on his back and shoulders after a workout then lifted him up chest-to-chest to rinse his hair.
The man is identified by prosecutors as Victim 6. He says he blacked out at the time of the encounter, when he was 11 years old.
Penn State police investigated after the boy's mother became suspicious when he returned home with wet hair but authorities ultimately decided not to bring charges.
On cross-examination, the man testified that he could recall no further sexual contact with the coach.
Sandusky has denied all charges against him involving 10 alleged victims.