Would believing the coming again of the Son of Man has already happened be considered heretical by the Catholic Church?
The short answer is yes, as it definitely goes against the traditional point of view of the Church and almost all Catholics would not accept it. It also goes against the Apostles Creed: "He will come again to judge the living and the dead."
The constant tradition of the Church believes the Second Coming of Christ has yet to happen.
Prophecy is generally not accepted as being complete until the events of the things prophesied have fulfilled their course. The Church has never accepted that the prophecies concerning the Second Coming of Christ have been fulfilled. In fact, I have never known any Catholics who have expressed this perspective. The Church simply believes it is yet to come. To believe otherwise is not heresy, but is not official stance of the Church.
There is a lot of speculation on this subject matter, even within the Catholic Church, but the vast, vast majority of the faithful hold that the Second Coming is yet to come.
It is the traditional view of Roman Catholics and Orthodox Christians, preserved from the early Church, that the Second Coming will be a sudden and unmistakable incident, like "a flash of lightning".[Mt 24:27] They hold the general view that Jesus will not spend any time on the earth in ministry or preaching, but come to judge mankind. They also agree that the ministry of the Antichrist will take place right before the Second Coming. - Second Coming