Hi, I can't say I know for sure what is causing your newborn to grunt as you describe, but I would talk it over with your pediatrician. I would be VERY cautious about giving a newborn a drug like Zantac....especially since you're not evern sure he has reflux. (And it also is just treating the symptoms and not the cause..)
It is pretty common for newborns to have difficulty adjusting to eating. Are your breastfeeding or is he on formula? If the latter, he might not be able to properly digest the type of formula he is on. There are formulas for babies with very sensitive digestive tracts. You can find them in the baby food aisle, or ask your pediatrician.
As to the sleeping issue, when our baby was 2 months old, the only way he would sleep for a long time was on his tummy. We would do this during the day when we coud keep our eye on him. For example, I would put him on his tummy on the couch while I was cleaning up or doing stuff in the kitchen. I was always able to see him, so was not concerned about SIDS. At night we used a special motion detector type monitor that would set off an alarm if the baby stopped breathing. So we actually let him sleep on his tummy at night, too. When on his tummy he would sleep 3 hours at a stretch, otherwise, he would wake up just like yours does.
You may want to try the tummy thing to see if it will work for your baby, but because everyone is so freaked out about SIDS I am not recommending it to you specifically.
Lastly, you might want to try co-sleeping with your baby in the early months, especially during nap-time. Dr. Sears has a grest section in his famous Baby Book about co-sleeping-even if you only do it periodically--and how new research is showing that the mother's breathing actually regulates the baby's breathing. Babies who were not sleeping well, or who had sleep apnea or asthma, had dramtatically improved breathing when they slept next to their mothers. It is very exciting stuff, actually. I highly recommend you read that section in his book. You might even be able to do a Google search like Dr. Sears _ co-sleeping.
I wish you all the best as you figure out the best way to make your baby (and you!) more comfortable. Just know that you are not alone, as many of us have been through these things!