If you choose InfoPath as a *somewhat* more user-friendly way to customize the form (supported until 2020 *but deprecated by Microsoft*), consider reading the recipes on bizsupportonline, there are lots of them, [your specific problem also solved][1] a [couple of ways][2].


 [1]: http://www.bizsupportonline.net/browserforms/filter-cascade-drop-down-infopath-sharepoint-browser-form-owssvr-dll.htm
 [2]: http://www.bizsupportonline.net/blog/2011/04/cascading-drop-down-list-boxes-infopath-2010-non-traditional-method/