Does my son in law have to be studying during his legal residency process?

I am confused by the advice that you have received and that perhaps all of the facts were not revealed in your question. However, a US citizen is allowed to petition for the spouse's child as that child becomes your stepchild by virtue of the marriage so long as the marriage was celebrated prior to the child's 18th birthday. As long as the child entered the country legally, you should be able to apply for him on form I-130 and he should be able to adjust his status on form I-485.Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.