Bottle feeding advice

My LO is almost 7 weeks old and in the last couple days he's started fighting his bottle towards the evening. He takes his bottle just fine during the day but after a certain time of day he fights it and cries because he's hungry. I haven't changed his nipples, formula, or the position he normally eats in, the atmosphere is pretty much the same as well. Has anyone else had this problem and if so how did you handle it? I just keep offering it to him until he finishes but it's been taking almost an hour for him to eat. Help!