I understand it would probably be uncommon in everyday Hebrew, but could you also say "מה כוונו" or "מה כוונתו"?
You could say מה כוונתו (the 'ה becomes a 'ת).
when can you say איזו in situations like this?
could you use that to ask "which is his intention", if there is an understood set of possible intentions?
I'd say that "איזו כוונה שלו" means "Which intention is his?" rather than "What is his intention". It does seem to imply a closed group of possible intentions.
אֲנִי אַף פַּעַם לֹא יוֹדֵעַ אֵיךְ לְבַטֵּא אֶת הַמִּילָּה כֻּוְּונָה. Kuvna??