The English Prisoner is in my opinion Bob’s dad, WOJ has it is someone we have been introduced to in series, and that may be ALL that very brief exchange was set up to do, at least at this point, because all it is, is a bloody introduction.
I think it is likely a historical Wizard from a fair way back, an apprentice of the Merlin, who was around about 1,000 years ago (given Bob’s age) and hiding out because it was he who found out how to kill immortals (which is where Bob got the info from)that would have made him contemporaneous to Mab when she was the Lady. I don’t think Mab is Bob’s mum, I think Lea given she is a muse, inspiring a man to and early death and he was so excited to see her in GP.
On that basis I fancy Michael Scot/Scotus The Wizard of the North, nice dubious date and place of birth so he could given a Wizards lifespan have been around a couple of centuries earlier under a different name. Had a habit of wearing a skull cap made of iron (the perfect contraceptive for a Fae muse, or to stop his mind being read) and has links in folklore to the Redcap.
We have been given hints of Mab’s past in BG, but Scotus could know firsthand, and how the relationships Mab had with Lea and the Redcap began, and much of the early history of the Court including Tam Lin, Harry’s most illustrious predecessor as Winter Knight and how he got away. He would be made of pure exposition, and likely Eb’s grandfather/ great grandfather/ great great grandfather making Bob Harry’s uncle several times removed. Would Jim set up a joke with its denouement a couple of decades later? yes, definitely.
Of course Jim could be lying and it is the time looped Merlin who is also Future Harry, his denials have I believe always one or the other, not both at the same time.