The boy took part in zanjeer matam, in which believers beat themselves with a wooden implement with chains and blades attached to it.
His father, 47, who also took part in the ritual at a Birmingham mosque, said he acted out of “love” and “devotion”.
He was given a six-month suspended sentence at Huntingdon Crown Court.Shia Ashura Ceremony (Daily Mail)
The man, who cannot be named for legal reasons, had denied being cruel to a child.
He is the second Muslim to be convicted of cruelty as a result of allowing a boy take part in the ceremony which commemorates the death and martyrdom of a 7th Century Muslim leader, prosecutors said.
Mozammel Hossain, defending the father, from Rugby in Warwickshire, said that his client now knew that allowing a child to beat himself was illegal in the UK and he would not allow it to happen again.
Mr Hossain added that it was a ceremony his client had also taken part in as a child.
The court heard that the man’s wife had not wanted her son to take part in the ritual in January 2007, when the man lived in Peterborough, Cambridgeshire.
The police investigated after she raised concerns.
Judge Nicholas Coleman, who watched footage of a ceremony during the trial, told the court that “adults are free to make their own choices but children must not be allowed to take part”.
“Not only did you allow him to participate in the ceremony, in my judgment, you actively encouraged him.