Wales need to distil that generational mistrust of the English from primary school like we do to avoid this sort of thing.
If he's lived there long enough and wants to play for England though I don't think he should be stopped. The "product of X" line is rubbish too whenever it's brought up. He's a product of his own talent, drive and hard work and is from Wales like, he'd have risen to the top there too.
Tangentially, a nice ruling for Leinster / Ireland amd Casper Gabriel. IRFU lawyers will be taking note.