Not quite sure what you mean. If the grandfather owns the aircraft his grandson does not have to return it. No problem if the aircraft is Australian registered. For a US registered aircraft, the Aus. PPL will not do.
LA to departure point to Iceland would be into Canada, which is no problem, but I guess you mean him to fly it to Iceland itself? Have a look at threads on ferry flying cross Atlantic, if he can get insurance for your aircraft, I would still value his life too much. - Let him build a couple of hundred hours in the US or Canada before in icing conditions and make sure he has an IR.
With regard to Iceland - Rome: look at a map.
I would like to suggest, with all due respect, that this granddad has very little understanding of the operation of small aircraft, licensing and geography.
Hopefully, the grandson will do lots more reading before being sent on a suicide mission with eyes shut. Luckily, there is no chance to obtain all permissions for this summer, so that the PPL will be 19 before he can go next year...
cheers, IP