Well, having an attorney would certainly make things easier for him, but he can do this himself if he wants to try.
Virtually every county has a law library. Your son will want to ask the law librarian to show him where the books on Pleading and Practice are located. Generally there are many volumes of Pleading and Practice books from which he can pick the proper one for his situation (Petition to Modify Child Custody/Visitation). While the librarian is typically not a lawyer, and cannot practice law, most are quite willing to assist visitors in locating the chapter in the Pleading and Practice book which he will need.
Once he has found the appropriate pleading, he will note that in addition to the sample pleading which contains all of the necessary structure for his Pleading, there may also be suggestions to make his pleading more persuasive. He will need to prepare his pleading in accordance with the suggested format in the book, inserting the facts of your case as he knows them to be. Then all he will need to do is to draft the pleading on a computer, print it out and file the original with the court while serving the other party, or their attorney, with a copy.
The court will set a hearing date and he can ask the court for a visitation schedule. Generally what also happens is that the court will suggest that your son and the mother meet with a mediator so that they can work out a visitation schedule. Once the court orders the Modification and approves the visitation schedule, the mother will have to abide by the court ordered visitation schedule.
You may reply back to me again, using the Reply to Expert link, if you have additional questions.
I wish you the best in your future,