Don't stress too much if he is otherwise happy and his diaper is wet or dirty enough during the day. That is the best measurement. Or if he is losing too much weight at his next dr appt.
My guess it that is a change of appetite or change of eating preference/pattern.
Their little appetites go up and down so much, sometimes month to month, sometimes day to day. He could have just come out of a growth spurt and doesn't need as much food now as he did. He could be hungry at a different time of day. Maybe wait to give him his first bottle, or give him the bottle first and then food an hour or 2 later in the morning. I would probably try to get more formula than food in him, but still don't stress.
My son is 2 and his eating habits and patterns have always changed every few months. A few months ago he wouldn't eat for at least an hour after getting up, before that he had to eat before drinking anything. Now he drinks 2 sippys of milk before taking a bite. It could just be a change of preference.