When was his hearing last tested? May be he should have his ears checked!
There is NO system adjustment for handset volume on the Partner and NO programming code. The only volume control is on each system phone, like the P34D sets they currently have. If that does not provide enough volume, he may needs a hearing aid. In this day and age lots of people have damaged hearing from all the loud stuff out there, things like Ipods and boom cars, etc.!
....JIM....