Hello, my lo never drank as much as their 'supposed' to drink. And she's happily at the 80th percentile in weight. She typically drank about 25 oz at 4 months old, some days less and some days more. She didn't get dehydrated or constipated.
Maybe he's teething. Mine got her first eye tooth at 4 months.
Also, what flow nipple are your using? When I switched to a fast flow nipple, it was like a miracle. She stopped squirming around all the over the place and took a good 5 ounces at once.
Good luck, Tarri