This is just a supplement to Maroon's answer which is far too long to be a comment.
In the raw manga, the original text is not akin to the English phrase "to go back" (as in, to return to the way things used to be; the metaphorical sense). It actually is a literal remark that he will be unable to return [to the real world] after destroying his gate.
Which translates somewhat more literally like,
If I could, I would do what Fullmetal did, and use my gate as the toll; but then, the path for me to return home would disappear.
Additionally, the reason Edward was able to give up his gate is, as Maroon said, because Ed and Al's gates are somehow connected. After Edward destroys his gate, he turns around and Truth points him toward Alphonse's gate, where Al is sitting. The two boys return through that to the real world.
Since Mustang could have done no such thing, it makes sense he would have concluded there to be no way to return once he had destroyed his gate.
Note: Please don't accept this answer; it's just here to support Maroon's; if you agree with it, please accept that one instead.~