As a general rule, it is not illegal for your son to do so.
However, be advised that the Court does not usually accept such correspondence unless it comes through the proper legal channels.
As such, your son does not need to have an attorney prepare this information. He can prepare it himself (acting in pro per), but it MUST be submitted to the court in the proper format and on the mandatory forms. Additionally, any opposing parties must also receive copies pursuant to local court rules and regulations.
Your son may want to check with the court to see if they have Family Law technicians available to help answer his questions. If not (or if they cannot provide as much help as he needs), then I suggest he make an appointment for a consultation with an experienced Family Law attorney.
He needs to get advice about his legal rights and responsibilities and find out how best to proceed. Once he does that, he can go ahead and do the paperwork himself or he can hire a legal document preparer to assist him (if he does not wish to retain an attorney).
He can also request the Court appoint an attorney for the children to represent their best interests, as well as ask that a Court Expert be appointed to evaluate the matter and provide the Court with a Recommendation regarding custody and visitation.
Good luck.
