Alternatives to "He is of the opinion"?
The idiom "is of the opinion that" is fairly well entrenched. Using it would not be ambiguous.
That said, I believe it's wordy.
Indeed, I'm of the opinion that it would better to simply say, "he thinks that", "he believes that" or "his opinion is". I think that works better. It is also my opinion that wordiness is unnecessary. I believe it's better to avoid the passive.
See what I did there?