1. If you are the primary applicant of an adjustment of status application you could do the business on the side, but if you take it up completely as your profession, then the job must be same or similar in other words you are trying to do your green card Supplement J through your own business if that's not your intention then you must have a full-time job and you can do the business on the side. Form I-9 is an internal form. It does not get submitted anywhere.
2. Yes he can start the business.
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