write back to the dvla and ask them for a list of acceptable medical centres in the area you would like him to attend and how they will go about funding the test, as it's not going to be free as he's not yet 60

a full medical costs from around £45 onwards and, in some cases, you will have to get it done by your own gp or go to a centre that tests hgv/psv drivers after they are 45, and they cost about £100