I wrote an article about this here: http://moviepilot.com/posts/2015/06/04/is-captain-jack-the-face-of-boe-and-why-is-he-a-head-3280496?lt_source=external,manual,manual
Jack's comment: "They called me the face of Boe" seems to be the only link between Captain Jack and the Face of Boe we're given in Doctor Who. It's possible it was a coincidence- he's called 'Bo', 'Boe' or 'Bow' face as a weird nickname; or a joke- either by the writers to the audience (because why not?) or by Captain Jack to the Doctor and Martha. If this is the case, we've got to wonder how it is that Jack knows about the Face of Boe, or at least knows that the Doctor knows of the Face of Boe...
The final possibility is what you were asking in the first place, 'Is Captain Jack the face of Boe?'. This means you're asking: 'Is Captain Jack destined to become a huge, sqidgy face in a tank?'.
Hmm. Well, we already know he's (supposedly) been made immortal by Rose, so that would explain how he managed to live to the grand old age of several million. The question of how he becomes a huge face could be explained by some (slightly sketchy) science. First up it's worthy to note that Doctor Who has never been that good at applying realistic science to the show, thus this may seem like a lot of Bulls**t.
Humans age becuase our cells are dying- wrinkles are a product of skin cells becoming less elastic-y and stretchy, various nasty diseases we tend to only experience- and die of-in later life are the product of the immune system going on a permanent vaction- etc., etc.... Therefore, if Captain Jack wants to live beyond the age of 150, he's going to have to find something a bit stronger than an anti-wrinkle cream.
As we saw with Cassandra, staying alive is not easy. After seven hundred and eight surgery operations, she had her brain put in a jar- seeings as the brain is arguable the most important part of the human body- and the last bits of her, erm, "body" stretched across a frame. It looks like she kept all 2-metres-square of skin (it is said that if you stretched all of a humans skin out like a towel it will be at least 2 metres square in length), and had her eyes and mouth left in the centre... for some reason.
Other vital organs appear to have been discarded by the Lady Cassandra, such as the heart and lungs, alongside a digestive and nervous system. I honestly have no idea how she managed to survive without them. Perhaps they were all in the same jar as her brain, and I just failed to notice it, or maybe our modern Scientists are wrong, and all we need to survive is a man who will hold our squirty bottle of moisteriser every time we demand it.
It would appear that Cassandra still has veins embedded in her skin, although I'm not sure if they're connected to a heart to pump them...
Back to the Face of Boe, why the hell is he a giant face?! Well, Captain Jack has admittedly always been one for style, and it certainly makes a good impression on your guests if you're a giant head. It makes it look like you have a giant brain, and if your sole purpose in life (even an overly-extended life) is to look wise, then that can certainly help your convincingness. Besides, who amongst us hasn't wanted to live out their golden years as a head in a tank?
Despite this, it's possible Captain Jack became a head out of necessity- he, like Cassandra, went down the eight-hundred-and-something-stupid surgery route, and- rather than becoming a piece of skin shouting "Moisturise me! Moisturise me!!!" every five minutes, he became an enlarged version of a shrunken head. Maybe he did the sensible thing and kept all his organs in there, or maybe they're behind him, plugged into a games control and playing plants vs zombies.
Of course, it's entirely possible that Captain Jack became a head against his will- as is so often suggested in other sci-fi franchises, radiation of the gamma, beta or really, really harmful kind is rumoured to be able to alter DNA. It's not hard to imagine the sometimes stupid and often risky antics that Jack could get up to in his spare time, and if one of them happened to expose him to radiation? WHAM, he's just got zapped into a giant. What the heck he did with the rest of his body I have no idea. Donated it to medical science? A modern art installation called 'The Body of Boe'?
Another option is that the natural process of ageing turned Boe into an immobile blob. Maybe all humans would melt into an overly-huge head if we got past the age of one million? Perhaps the dude got so many wrinkles he seized up, only speaking with his mouth for special occasions such as telling the Doctor "You are not alone"- he speaks telepathically, if at all, before his final appearance.
Whatever happened, it's safe to say that Captain Jack could be the Face of Boe... However weird his transformative period was.
NOTE: some fan-theories state that Jack is a head becuase of the headless monks.
Further note: Jack was born on the Boeshane peninsula and nicknamed the face of Boe for his modelling career.
Further Further note: Russel T Davies has never wanted to link the two charachters together: http://en.wikipedia.org/wiki/Face_of_Boe