Yes, he is. Both are descendants of Henry Percy, 4th Earl of Northumberland.
Thomas Percy was the 4th Earl's great-grandson, whereas George Percy is the 4th Earl's (female-line) 16th great-grandson, making them third cousins 12th times removed.
George Percy's descent from the 4th Earl of Northumberland:
- Henry Percy, 4th Earl of Northumberland
- Henry Percy, 5th Earl of Northumberland
- Sir Thomas Percy
- Henry Percy, 8th Earl of Northumberland
- Henry Percy, 9th Earl of Northumberland
- Algernon Percy, 10th Earl of Northumberland
- Josceline Percy, 11th Earl of Northumberland
- Elizabeth Percy, Duchess of Somerset
- Algernon Seymour, 7th Duke of Somerset
- Elizabeth Percy, 2nd Baroness Percy
- Algernon Percy, 1st Earl of Beverley
- George Percy, 5th Duke of Northumberland
- Algernon Percy, 6th Duke of Northumberland
- Henry Percy, 7th Duke of Northumberland
- Alan Percy, 8th Duke of Northumberland
- Hugh Percy, 10th Duke of Northumberland
- Ralph Percy, 12th Duke of Northumberland
- George, Earl Percy