If libraries and Family History centers ever reopen, many abstracts of court, land, and probate records for Frederick County are available. You might look for histories of Winchester, Virginia also. And just an FYI, although his land grant was from the Northern Neck proprietary it wasn’t what’s called the Northern Neck, several counties in eastern Virginia between the Potomac and Rappahannock Rivers, near the Chesapeake Bay. Except for that land grant he won’t be found in Northern Neck records. Frederick County was created from Orange in 1742.