How awful for your son and your family. So sorry. Marking "disabled" will not have any effect on his tax return--there are no extra credits or deductions for saying someone is disabled, except when you have a dependent who is over 12 and you have to pay someone to take care of them so you can work.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
