need advice please! my son is about to be 3 months old (sunday). since last week he hasn't been eating well. I figured…

need advice please! my son is about to be 3 months old (sunday). since last week he hasn't been eating well. I figured it was his 12 week growth spurt as he was also really fussy. but the eating trouble is still going on. he usually eats 4oz every 3 hours. now it's hard to get him to drink 3oz. at night he usually will still drink 4oz but during the day all he wants is 2 or 3. I'm worried that he's not getting enough and don't want him to get dehydrated. what do you think is going on and how can I get him to drink more?