Well, if you feed him by about 8 in the morning or earlier, then he should need to potty by about 10pm or so. You could encourage him along by giving him a bit of exercise around that time/before bed. Nothing like running around to get a dog's bowels moving LOL And if you know he's pooped before bed, you can be reasonably safe in ignoring any wish to go out before morning, just in case it is largely habit.
I wouldn't recomend one feeding a day. A lot of boxers get upset stomachs if it is too long between meals. Feeding large quantities of food at a single sitting also increases the risk of bloat. So from all perspectives, feeding twice or more a day is better for him that just once.