Could an alternative be "il joue en plein air" ?
Less common, but it would work.
why do i not use est here? does it have to do with to play being a transitive or intransitive verb?
Duolingo has never allowed us to do that. It's always been il mange, il doit, il lit, etc. For as long as I can remember, "he is whatevering" is always answered in the form "he whatevers" when translating.