Just some ideas, no records:
1. Could it be that Benjamin Fullum went off to fight in the French and Indian War and just stayed in what would become Washington County? The war was fought in western PA, along the Ohio River, etc., I believe. Maybe he never went back east to Frederick, VA.
Does it say what military unit/regiment he served with, or under what captain, etc? The muster rolls might tell you that. You might be able to learn where exactly he fought. Google the regiment and/or the captain + the war and/or the county.
Are there records from the future Washington Co area for the time period 1758 to 1783?
2. Is he mentioned in any of the county histories for that area? Google, for example, Washington County Pennsylvania county history, biographical portraits, early history, early settlers, etc. Include the state. There are a lot of Washington Counties in the US.
Just googled it: there are 31
3. Were there other families from Frederick VA, in what would become Washington County PA? That might add to the evidence that the Benjamin Fullum in Frederick VA was the same as the man in Washington Co.
4. Were the variants on the 1760 census of VA close enough that they could be him? Then again, the war didn't end until 1763. Were men in the army on the census? Were the variants in Frederick Co? Could they have been his family members?
5. Were there other Fullum families in Frederick, VA? Were they white or black? Does knowing that help identify Benjamin? Could he have been white and married a free black woman? I think the id of color in the column on the census records was in the eye of the enumerator, not how the family viewed themselves, but I might be wrong.