Is that true? If so, is there really a chance of him actually winning?
No. By all estimates, if the Republicans nominate Trump, they lose. Either Democratic candidate beats Trump in hypothetical matchups in the general election.
He's winning (in the
primaries) because there are no other republicans with national name recognition that aren't hated. Cruz has made real and lasting enemies in the Senate, even among his own party. Rubio was being groomed for the job, but he had a chance back in
2013 to break onto the national scene, it seems to have gone downhill from there. Jeb Bush, brother of GWB, seemed to rely on speaking spanish and his family name to get elected, which didn't fly with the xenophobic elements of the republican party. No other candidate broke 10% in a primary, besides John Kasich in his home state, which isn't very impressive considering he only had to beat a new Yorker in Ohio.
It's just a shitty year for the republicans. No one with a conscience wants to be associated with the party

Edit: BTW my source for those hypotheticals is
RealClearPolitics which aggregates polls with biases in both main directions (ie, Fox News as well as Bloomberg).