It sounds weird. The issue here is that bliver denotes an action, while være is for conditions, so "Var han behandlet?" sounds like "Was he a treated man?"
A nicer example might be the following:
- Var lyset slukket? - Was the light switched off? You're caring about the condition of the light.
- Blev lyset slukket? - Was the light being switched off? You're caring about someone having actively switched off the light.
I don't think as blev is præteritum, the equivalent of past simple in English, so it expresses an action with no ripercussions on the present.
If it was "Har han blevet behandlet?" it implies that he is still beeing treated so I guess it would do it. Or "er han behandlet?"