9 weeks old baby drinking lesser

My 9 weeks old baby used to finish all his milk and gets hungry every 2 to 3 hours. Lately, he has been drinking much lesser and at longer intervals, sometimes up to 4 hours. He seems less interested in his milk as well and will spit out his milk and cry when we try to make him finish his milk. He is still acting normal other than that. Has anyone experienced this before? I am quite concerned as he is drinking lesser than the recommended amount for his weight and his weight gain has slowed down. Thanks