I see no particular reason to disbelieve the honesty of his statement(s) but obviously trusting him is something that requires some caution. He was basically still a child when he set off on his fanciful and ill-fated path. By now a lot of reality will have made a dent/impression in/on his fervour/mind. If I imagine myself as a (passionate, easily influenced, etc.) romantic ideologue at that age, say some sort of nationalist, then I cannot swear that I might not go further than I should in trying to bring about my type of utopia ("democracy", "freedom", "control", etc., etc.). Right through history there are countless examples of misguided youths of all sorts, IS recruits are the latest and most modern manifestation of such folly (at the other end of the folly spectrum there are those who confine themselves to ranting on an internet forum in their old age).
It should be relatively easy to examine his willingness/intentions when it comes to speaking out against Muslim extremism. From what he is reported to have said it strikes me this man is very possibly at least of average intelligence, not a vacant head - which is likely exactly why he is in his present predicament. This business with revoking of nationality is of course, like so much else from that direction, simply for convenience on the political stage, there is no deep thinking behind it.