He should be able to do a Solo 401k or a SEP. Solo 401k probably allows him to contribute more: $18k + 25% of salary = $53k max. SEP is whichever is less: 25% or $56k.
Yes, this allows him to shelter some from taxes, but it looks like his contribution to the retirement plan in the year he files the FAFSA would count as untaxed income. This stuff is confusing, so do your own research.