If he wasn't required to file taxes for a year (i.e. if he had no income), then he can provide a signed letter stating that he wasn't required to file because he had no income for those 3 years.
An employment letter + pay stubs would be the best way to show his current income. That said, that being his only job in at least the past 3 years might be a concern fore the IO. Barring sufficient assets (3x the required income level), it may be wise to use a joint sponsor just in case.
A joint sponsor can sponsor as many people as they are capable and willing to do so. It's not a red flag or anything.
tkse7en777 reacted to CEE53147 in AOS. USC has files no taxes, however recently started working.