1. It was he that(who) signed autographs for the fans after the show.
2. It was him that(who) signed autographs for the fans after the show. (acceptable in informal style)
3. It was him signing autographs for the fans after the show.
While I understand the ‘him’ in no.2 above is acceptable in an informal style of a cleft sentence, I’ve heard a few local persons here insist that no.3 is also acceptable, since ‘the subjective relative plus a verb’ could be reduced into ‘-ing’ form.
For the issue - whether this reduced cleft sentence in no.3 as a variant is in fact being accepted by natives recently in informal or conversation style, I would invite your opinions.
deepcosmoswhether this reduced cleft sentence in no.3 as a variant is in fact being accepted by natives recently in informal or conversation style
I don't think this kind of cleft is necessarily informal (or particularly formal either), and it is certainly not a recent feature of English. The examples below (found online) are all from texts that are more than 100 years old!
It was him looking down dubiously. (1916)
It was him doing the shooting. (1907)
It was him going in. (1917)
To my ear, the first example (It was he who ...) does sound very formal. (AmE)
Nowadays American English uses "He was the one who ..." most often to express that idea.