We have to keep in mind that this is a 5.5 month old pup too.. they usually cannot hold their bladder or bowels for eight hours everyday. If there is no way you can arrange for him to be let out during the day I would confine him to an area of the house (preferably with laminate or tile floors for easy clean up

). Make it a small area with nothing that he can chew up or hurt himself on. A gate in the kitchen or bathroom would be good. there are also play pens you can buy, though I don't know how well that would hold a boxer pup.
In a few months he should be better able to hold it and then you can leave him crated. I would still work on crate training if you aren't already though, you can use it for shorter absences like grocery shopping or going out.
Good luck!