advice for babies that fight naps

Any tips for babies who fight naps? My 15 week LO screams anytime I try to get him to nap. It doesn't matter if I catch him before he gets too tired or if he's already started fussing... he screams 5-20 mins until he falls asleep. The only thing that will soothe him is a bottle (he's formula fed) but he's not hungry and will get a belly ache after his nap and I don't want him to think he has to have a bottle to fall asleep so I don't give him one. I always let him do something more relaxing about 30 mins before he should nap, like set in his swing, so he will calm down... I've tried routines but it doesn't really help either. Also, I need to be able to get him to sleep without fighting if we are out and about. ANY tips would be greatly appreciated! (BTW- at nighttime he doesn't not scream or fight it at all, only for naps)