Almost 1 year old not wanting to drink formula

Hey everyone!

My son turns a year old on Saturday and I’ve been a bit worried about him. He’s not drinking as much formula as he usually does and it’s been going on for a few days. He eats a bit of food and will have at least 8 ounces before bed and in the morning but during the day will only drink 5 or 6 ounces. He is staying hydrated because I give him little bits of baby water in a sippy cup. My question is is this something to be worried about and call his doctor or is he actually just starting to wean himself off formula? Thank you in advance for any advise!