If he put a period after "...can't spell" I'd agree with you: >A guy who writes I could of been a contender can't spell. Sure, he knows how to say grammaticall

If he put a period after "...can't spell" I'd agree with you: >A guy who writes I could of been a contender can't spell. Sure, he knows how to say grammatically that he could've been a contender (and who knows, perhaps in some realm other than writing he could've been). Conversational and grammatical.