The t-stat is on 90 but what is the temperature? you do have a thermometer to read the temp correct?
If he is not in his hide much it's because he does not feel secure enough in them, at that age especially in a glass enclosure BP will hide unless the hides are not tight enough and do not provide adequate security.
As far as food by now he should be at least on small adult mice if not adult mice, BP are started on hoppers but quickly move to adults in a matter of weeks, so this could be one of your issue as well.