Is waking up at night once to use the washroom a red flag for men's prostate gland?

Waking up once a night to use the bathroom is probably nothing to be concerned about. (Many nights, I get up to use the bathroom, and I don't have a prostate!) If it were several times a night, then I would be concerned. The fact that he uses that bathroom at night may have to do more with how much fluid he is drinking, especially later in the day (is he well hydrated?). I also don't think you have to worry about him ejaculating 5-6 times a week. As far as I know, this is normal and healthy. In fact, this frequency will probably help to prevent problems such as erectile dysfunction later in life. However, if you have more reasons to be concerned, have him ask his doctor at his next office visit.