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Hello. Thanks for the question.
This is a really long time to have intermittent colitis. I would really start thinking about getting a colonoscopy done to look for a rectal tumor, ulcer or polyp. These are best treated surgically by removing them. If caught and removed before they metastasize, we can get surgical cures.
If you want to just try a round of Metronidazole first, that would be ok. I would also give 1 tablespoon of canned pumpkin (fiber source) with his meals over the next few days. I would keep the colonoscopy in the back of your mind, though, in case the signs return. If they do within the next few months, then I would get the procedure done and make sure there is not something else going on.
I hope this helps, let me know if you have any other questions.