Advice for fighting sleep

I’ve got a 14 week old baby up until very recently has been such a good sleeper during the day for nap times. He’s absolutely great on a night, will go down to sleep at 9pm and sleeps for 7/8 hours.
During the day is another story. He fights so hard to stay awake at nap times and we often have about 40 minutes of kicking and screaming before he will finally go to sleep.
He doesn’t have set nap times I just tend to go by when he is starting to show me signs he is tired. He’s breastfed with a bottle top up due to severe reflux which was causing him to lose a lot of weight. He is on omeprazole for the reflux which is maxed out for his current weight. Usually when he was tired I could put him on the breast and he would go straight to sleep if all else failed, now he just screams at the breast too. This is my second baby and I never had any issues with my first regarding his sleep.

I know babies can go through a sleep regression at around 4 months old, could this be catching us a little early?
Obviously the weather we’re having at the minute isn’t helping but I guess I’m just asking if anyone has any tips that might help.