Unlike the UCAS system for undergraduate applications, there's no specific cut-off date for applications for post-graduate courses.
This is from the Sheffield University FAQs for post-grad courses:
"What are the starting and closing dates for applications?
There is no specific starting date for applications. Most courses start at the end of September so we would recommend that you apply by the end of August. However, you should try to apply before this to allow plenty of time for your application to be processed and for a decision to be made."
So, your son would seem to have plenty of time to get his application in. (Sheffield's Open Day for potential post-grad students was only 5 weeks ago, so there will still be plenty of people making their minds up about where to apply).
Individual university departments are free to determined their own entry requirements for post-graduate study. They normally require a qualification equivalent to a British good honours degree. Your son's qualificatiuon might be something that the UK university of his choice already recognizes. (The only sure way to find out is to ask them). Otherwise, your son might have to get his US qualification validated by UK NARIC. They are the official body for this task. They charge a fee (�40, or �36 online) for this service but you could probably get some initial information for free by phoning 0870 990 4088:
http://www.naric.org.uk/default.htm
Apart from teacher training, there is no government funding for post-graduate courses. The government's advice on obtaining funding for such courses can be found here:
http://www.directgov.gov.uk/en/EducationAndLea rning/UniversityAndHigherEducation/Postgraduat es/DG_10