Hi There,
There are several things here - zantac or ranitidine (same med) is a good first port of call but not everyone uses it first off and just goes straight to omeprazole.
The omeprazole solution really doesn't work as well as the granules. The granules can be given in a tiny amount of pureed pear or something - the amount is so tiny that it really really isn't counted as starting solids.
There's info about this and the dosage chart on the Crying Over Spilt Milk site.
http://www.cryingoverspiltmilk.co.nz/MedicalTreatment/MedicationwithDosages.htmConsidering a low dose would be 0.7 - 1.5mg per kg I wouldn't be confident that your GP's dose will be effective for a 5kg baby!
On the web site there is a chart you can print with dosages so you might want to do that and go back to the doctor with it.
Best of luck!