Have had a further update today. The blood clot was not resolving so R had an operation to remove it and has been kept in an induced coma for the past couple of days. Medics now feel that he is well enough for them to start reducing the medication and allow him to wake up. It's still very early days but at least a step in the right direction.
The good news is his age. At that age, the blood is less viscous and collaterals can develop earlier (and on a temporary basis if required) so this will enormously help him get over this. I'm not 19 so there was less going on in my favour but, as the consultant told me much later, residual fitness helped me immensely. If your nephew has fitness AND youth going for him, he'll sail through and be back to normal (like that TdF rider who had a fractured skull this year but rode the 2010 event) 'ere long.