Login can be a verb, we just haven't collectively decided how to conjugate it. Correct in language is usually more a function of convention rather than functionality. There just aren't rules to handle this. It can either be "he logs in" where we just revert to the two word version, which appears to be the most common approach, or you just conjugate and recombine. "He logsin". This is harder to recognize and pronounce so I prefer the former. Eventually, the matter will get settled by convention and then grammar nazis will nitpick people for not knowing the rules for conjugating verb phrases correctly.