“ ‘Son of man’ is a title that belongs to Christ. Why is it also used for Ezekiel?”
Ezekiel not only had the title before the Lord Jesus, he had it used to describe him many more times than the Lord Jesus, 93 in total. More importantly, it was God himself who chose to address him that way, though Daniel is also called a “son of man”. But Ezekiel and Daniel are not the only places you find the phrase in the Old Testament; you also find it in Job, Numbers, the Psalms, Isaiah and Jeremiah.
What can we learn from the fact that both Ezekiel and Daniel had the title earlier, and Ezekiel more frequently? Not much, probably, except maybe not to measure spiritual importance by such metrics.