Hi ES Barnes
Thank you for your question, it is quite complex and we cannot give you an accurate answer as we do not have all the facts. We would advise you to contact your local Welfare Rights Organisation;
advicelocal.uk where they can take all your son's circumstances into account.
Reading your post I would say that your son will have to go through the assessment phase as his NS ESA had stopped when he applied for UC. Therefore to answer your question I would say yes he needs to complete UC50, usually DWP do not grant extensions to complete the form.
Gary