He should get moving if he wants to be the next President.
To be technical it is even less likely that he will win, if he doesn't run. ;-)
Of course nothing is a sure thing, but Thompson still has a good chance of garnering a plurality, if not a majority of Republican votes in the primaries. One reason for that is his plain spoken conservativism, but another is that he is the best candidate to put up against any of the Dems.
Even then his election to the Presidency isn't a sure thing. The Dems seem to be polling in the majority right now. But Thompson will garner a good number of cross over votes the way Reagan did. He is the best hope right now of keeping the Reagan Revolution alive!