You've omitted the first part so all you have is a guy called Jack who has said that he will be attending the ceremony on the following day. (I think - you haven't told us what he actually said, so that makes it difficult to decide how it should be reported. We only have your reported speech to infer what he said, and then tell you how it should be reported!)
If he said "I will be attending the ceremony", and the ceremony hasn't already been held, I'd write "He said he will be attending the ceremony".
If the ceremony has already been held, I'd write "He said he would be attending the ceremony".
I agree with your colleague. In general it's OK to write "He said he will be/would be ..." but to assert "He will be attending..." is a different thing.
If it was someone important, or if you're reporting officially, it's necessary to write such things as "The Queen will attend the performance". But if it's just Jack who might show up, and might not, "He said he would.." is better.