His body is most likely experiencing spasticity. Here is a good link from this site with more information:
http://www.christopherreeve.org/atf/cf/{3d83418f-b967-4c18-8ada-adc2e5355071}/Spasticity%207-08.PDF There are links in this article and you should be able to explore treatment options that are commonly available through some of the links.