Increasing Water

My son typically drinks what he is “supposed to” for his age - about 8 oz a day, sometimes a little more depending on what we’ve done that day. But then there’s days every once in awhile (1-2 times a week) that he doesn’t seem to want to drink as much - usually 6 oz but sometimes drinks as little as 4-5. He seems well hydrated as far as wet diapers and such, but he does have occasional episodes of harder poop (not so bad that he’s constipated or is in pain, but enough that it’s definitely formed, more dry, and I can tell it’s harder for him to go). The days with harder poop seem to follow a day where he didn’t drink at least 7-8 oz. Has anyone found anything that works for encouraging their kiddo to drink more? He has access to water whenever he wants it and I offer his cup to him often. I’d like to see if there’s something else I could try on those days where he doesn’t seem as interested in drinking his water.