When he told Vito in the hospital that he was with him, he was telling him that he, AS A SON, was with him. He was not telling Vito that he was with him as a prospective mafioso ready to take part in the family business.
I hear him as saying "I'm with you now" with emphasis on the "I'm". As in not these useless clowns who all managed to get themselves arrested. It's a son reassuring his father and has nothing to do with future mob activities. It was strictly in the here and now.
