What to do when baby fights sleep

Here lately my LO wants to stay awake a lot longer after he eats. He'll play and be content for a little but after about 1.5-2 hours he gets SO tired and SO fussy. I try rocking him, walking with him, etc but he just starts screaming. The only thing that calms him and gets him to sleep is another small bottle. (Normally he would eat every 3-4 hours). I usually just give him 1-2 ounces and it does the trick - I'm usually helpless at that point so I don't have any other solution. Like after I give him the bottle he's dozing off, if not asleep, within SECONDS! But I don't want him to get in the habit of having to go to sleep with s bottle!! What works for you moms?? He's only 6 weeks old so maybe this is okay and I don't need to worry about this until he is older?? I just don't want him to cry just to get a bottle or think he can go to sleep without one.