Hi there,
My DD started on Zantac when she was about 4 weeks old at 0.7ml. We noticed a very slight difference in her behavior after 3-4 days. That said, we went to a different pediatrician (long story) and he upped her dose to 1.1ml. That made a BIG difference and we saw the change after one day.
As far as I know, the only documented side effect of Zantac is diarrhea/constipation. The fussiness and tiredness are probably indications that the reflux is bothering him more. If it’s not being managed then the damange to his esophagus will continue, causing more pain. If he’s in pain then he’s probably not sleeping as well as he could (that was definitely the case with our DD). HTH!