Other tricks: 1/2 hr before bedtime, have him go to the bathroom. 1/2 hr later, have him go again. It's called a "double void" when at the urologist's office, and if the problem is physical, and his bladder is not completely emptying, then this may help. (I had a daughter with bladder problems who saw the urologist regularly from age 5 until age 10 when her body matured enough to solve the problem)
Trick #2: Be sure it isn't a bladder infection. When our daughter went off the low grade antibiotics she was on for regular infections. she would get an infection. It was always only a matter of time. So we told her that if she'd slug a shot of cranberry juice every evening at supper time, we wouldn't put her back on the pills that she hated. She hasn't had a bladder infection since. And she hasn't wet her pants without having a bladder infection.
I would like to know how many nights (of having the wee pad alarm go off) it took before your nephew started to wake up to urinate. . . . it seems kind of backwards, because the alarm goes off AFTER the fact, not in time to get you to get up and go.
So, a cheaper method would be to wake him up yourselves, before you go to bed, and have him go to the bathroom.
One other remedy from the urologist's mouth: no soda. More water.
Good luck. It's frustrating. We were really lucky that our daughter was small, so when she went on overnights, she brought a pull-up with her, and changed into her pj's privately. Then she'd just leave the pull-up (wet or dry) in her pj's to come home. It made her overnight bag a bit smelly on the return trip, but she didn't get embarrassed and she didn't wet anyone else's sheets or mattresses. :-)