Young Boy Apparition
This is not my theory–I read it on DarkUfo in the comments section, attributed to “Prenden2”:
“On the subject of ‘young Jacob’, seeing him slightly older inspired me to come up with a new theory; what if everytime MiB kills Jacob, he regenerates over a period of time and, when he reaches the age at which he first gained his powers, he regains all his control over MiB? This would force MiB to have to work to a strict timetable, he could only have a brief window-of-opportunity to escape the Island before he’s back under Jacob’s thumb. Thinking of it this way, the Loophole could be MiB’s plan to essentially knock the pieces off the board while Jacob is powerless; while he’s unable to help the Candidates.
Going by this theory, MiB becoming flustered each time he sees the kid could be explained by each apparition reminding him that his window-of-opportunity to escape is becoming increasingly shorter.”
This is such a good explanation for who the young boy is–when we first saw him, he was smaller and blond, now he’s taller/older/brown-haired. I checked on IMDB–it’s the same actor–Kenyon Duty, so I think it’s the same character, as Prenden2 has stated. He is the only person that MIB is spooked by.