He is rewriting the site. Why? Because he wants to add a bunch of new features that would take longer to hack into the old system then to rewrite an entire new system. He is open to learning some new things, hence the language switch.

He isn't switching for the sake of redesigning, I don't know how you got that impression.

Maintenance time is a key factor here - he rarely looks at the code, so PHP-spaghetti (we are talking '99 PHP, pre-OO and basically impossible to not create spaghetti) is getting unbearable.