Reflux mommies, help me!!!

Our 13 lb 2 mo old has been on Zantac since about 3.5 weeks.  Last week, it just stopped working, even with almost double the dosage than he'd previously been on.  We are now (as of this morning) on 5mg of Nexium given once in the morning.  Tonight, his grandparents were babysitting while we went to a wedding and ended up having to call us to come home bc he started screaming (first in hunger) and then in pain during his bottle lasting about 90 minutes.  We still had Zantac so I told them to give a dose of it.  It seemed to calm him almost instantly and he took most of his bottle.