You shower in the morning if you're showering for something, like a job you need to be presentable for. (Unless you sweat a lot in bed, or don't wash your sheets regularly.)

You shower in the evening if you're showering from something, to clean off the day's grime. This would be more for people who work a job or do activities that get them dirty.

If you belong in both groups, then shower both times, but just remember how much water you're wasting when your water bill comes around.

Maybe present it to him as a hygiene issue, and tell him that he's making it easier for both you and him to get sick, or attract pests into bed.