As this is a saying, is that why he is is not acceptable?
I tried 'he is as hard as an iron' and failed. I tried 'he is as hard as iron' and succeeded.
I'm guessing this would be a phrase requiring some context. Taking a bite of a badly baked bannock, you would definitely use "it"... looking at muscular men at the beach, "he" should be acceptable...lol.
Nobody told me that learning Gaelic would involve IRN BRU, a mouse in a loaf, salted gannet, or looking at muscular men at the beach. I am as happy as a shoe.