This is a tricky area for most newcomers. First of all, if his I-94 expired and you have not filed AOS he is deportable, so be careful and file ASAP. Second, most DMVs require a still-valid I-94 as proof of him being lawfully present. The problem is, even after you file AOS he will not be lawfully present (in the USCIS sense of the words), but rather in period of authorized stay as @Lil bear pointed out - that is not the same thing as lawfully present and he'll probably have to wait for EAD/GC.