You should talk to your son's attorney and see what the allegations are. Your son's attorney should get a police report at some time and you should find out what is going on.
Attorney Jason Chan
77 Franklin Street, FL3
Boston, MA 02110
disclaimer: I am not licensed to practice law in your state. Please be aware that this is for informational purposes only.
As I explained in my response to your other posting at http://www.avvo.com/legal-answers/question-on-robbery-charges--138651.html I believe the PA statute allows the threatened use of a weapon to suffice to establish the 'serious bodily injury' element of the offense. The fact that it was a toy gun, will most likely not be a determining factor in whether the State can establish the threatened use, at least as far as the robbery charge is concerned. As my colleague suggested, you need to discuss these issues with your son's lawyer, or another lawyer licensed to practice law in PA. Good luck.
I do not practice law in your State. This answer is provided solely for informational purposes only. This answer does not constitute legal advice, create an attorney-client relationship, or constitute attorney advertising.