Tom was also a son of Henry VIII.Considering what a horny dude Bluff King Hal was, this makes perfect sense.
Tom is a grandson of Henry the VIII.In terms of age and opportunity, this also works.
- Not really. It assumes either Mary or Elizabeth had a child without anybody knowing, which is pretty implausible.
- The OP probably just means that one of Tom's parents is a bastard child of Henry's. We never see or hear about either of Tom's grandfathers, so it could be possible.