4 month old won’t nap

And I’m not exaggerating. He will not sleep a wink during the day even though he’s way beyond exhausted. He sleeps 8 hours at night but that’s it. I’ve tried rocking him, taking him for car rides and on walks, white noise, dark room, his swing and NONE I repeat NONE of that has worked. I even make him do the thing he hates most, tummy time, and tire him out and he still won’t go down. I’m at my breaking point. I know he needs more than 8 hours. But what do I do?! Why is he like this? HELP PLEASE 😩 he sleeps fine in his crib at night. He has GERD but we are managing it and it’s currently the best it’s been since he’s been born so I’m just lost.