Hi Colin
There have been a number of threads regarding this website -
http://www.rootschat.com/forum/index.php?topic=713127.0 - for example. Personally I am wary of any paid site that does not give free trials or introductions for a small amount.
WW1 records were first put online by Ancestry who had a contract with the National Archives. Findmypast appear to have a similar deal now. The records are Crown Copyright so will only be available to other sites if they have an similar agreement. It may be worth contacting the NA and asking for details of any companies that are allowed to publish WW1 individual records - known as WO363 and WO364 in the NA.
Here is a quick introduction into the organisation of medical care during WW1 -
http://www.1914-1918.net/wounded.htm . The key is to discover where he is buried. The further back from the front the higher up the treatment ladder he would have been sent. The cemetery descriptions on CWGC often tell you the nearby hospitals. If his body was not recovered then you know you can stop looking for hospital records.
If you identify a particular hospital then a question on their sister site, the Great War Forum, should confirm the availability of records.
Death certificates are usually a waste of money. In almost all cases they only repeat the cause of death - KIA etc, and do not go into details
Ken