Hello Paul, I didn't know your cousin personally however I registered on this forum to give you what little info I have, which isn't a whole lot but thought you might want to know anyway:
What I remember about that accident is, I believe it occurred on the High Road, a stretch of road from Dunoon to the US Navy Base on the Holy Loch. At that time the base hadn't been there very long, I believe the tender at that time was the USS Proteus. This was the first fatality involving any serving US personel and given his young age it was especially tragic. Over the years there were a few serious accidents involving US Navy guys and their dependants, I think due in part to the fact that they were not used to driving on the opposite side of the road!
My condolences to you and yours, hope your able to make some connection.