tonic water contains quinine, wuinine can in few cases help night cramps but it needs to be in much bigger doses than is in tonic water, you need to get it from the doctors. Its normally to do with body salts, if he drinks too much water he could be diluting them but if he isnt drinking enough then then they are concentrated and could be causing them. Magnesium and zinc deficiencies are a common cause of cramp. Eating a banana 2 hours before he goes to bed may help. Also stretching out the calf muscles by putting his tiptoes on the stairs and then dropping the heel down will help too.