Are Karo syrup & water safe for baby?

Hello mommies, have a few questions once again. My DS is 7 weeks old and he is having to take karo syrup to help him poop. He does not have hard poop but he just spends ALL day struggling, pushing and even getting really red because he tries so hard to let the poop out. The Dr. told me to give him karo. So I did. I gave it to him with his formula two days ago. Once in the a.m. and once in the p.m. However after the second time when it was time for him to drink again, he gave us a hard time to drink the milk. I am afraid he got use to the karo syrup and did not want that bottle because it did not have any karo in it. Can this happen in a baby so little? Can they already know a difference? The next day (yesterday) I gave him the karo in one ounce of water each time. However, I have read and heard that you should not give a baby water. What do you ladies think? Also today he has had two feeding so far and he has not wanted to finish all his milk. Can it be that he is filling up on the ounce of water? He also has silent reflux so they have him on Zantac. He would gag alot and he would scream and cry every time he would drink milk. We have been dealing with this for three weeks now. When he started on Zantac a few days later it got a little better but then again yesterday night and today he is back to screaming in pain when he is drinking. He also kicks is legs out a lot and gets really stiff while drinking. He spends a big part of the day whining and kicking his feet a lot. He is also still doing the pushing and struggling to poop even though he alteast does poop thanks to the karo I am sure. Can it be that he also has colic. He does sleep well at night, only waking up once to feed. However for the past three days he has been waking up at 2:30 to feed then again about 2 hours later he wakes up complaining and kicking his feet and pushing and all that stuff. He spend about 2 hours like that until he finally lets the poop out. Is it that I need to give the Karo syrup more days to take effect? Could it be the water I am giving him? Or can it be the formula. He has been on Genltease for 2 1/2 weeks. Should his stomach be use to it by now. I spoke with the doctor on Monday and told him how that Zantac has been helping because when he does gag or even spit up now atleast he does not cry. But I mention to him how he is still fussy and struggling to poop and he told me to give him karo and let his stomach adjust to the formula that it will eventually adjust and I wont have to give him the karo any more. Do you ladies agree? I just hate seeing him suffer and hearing scream when I am feeding him.