I strongly encourage him to use the IRS data retrieval tool to import his tax return into the FAFSA. There will be a question after doing so about income earned from work; his answer will be 0. Using the IRS DRT makes things much easier … he will be allowed to skip some questions if he uses it.
If he can’t or won’t use it, just follow the FAFSA instructions carefully & use the information on the corresponding lines of his tax return.