Does it make sense to let a colleague know about his repeated misspelling?
Let it go, while the typo is limited to emails. Emails are frequently written hastily and certain typos can be common or made on purpose to type less, or even for fun (I do sometimes this in my language, to mimic certain dialectal expressions).
If, instead, the typo appears in, for instance, a paper or other public document you're jointly writing, then correct it as you would correct any other typo in the paper.