I don't know if you are breastfeeding or not, but you may consider removing dairy from his(or your) diet. both of my sons had reflux and dairy played a large part in it. Babies are commonly sensitive to milk protein. Dairy can also be constipating. You also may have to give the antacid a little more time. His little esophagus may need time to heal as well. Another thing to consider is that he has now gotten himself into an unfortunate pattern of night waking, so even though he may feel better, he is used to getting up during the night. Also, at 5 mos he may actually be hungry at night. Some babies eat more than others and feeding more frequently can be soothing to a baby with reflux. I would give him another week then try to give him a chance to soothe himself back to sleep if he does wake at night. This way you can be sure he's not waking due to discomfort. As for the constipation, try giving him a little prune juice until his body hopefully adjusts to the antacid. Good luck!
A.