EBF baby won't take a bottle of formula

My 5.5 month old EBF son refuses to take a bottle of formula! Help!

I'm in a wedding next weekend and my in-laws will be watching my son for the day. I've tried pumping but never had any luck, literally will pump for 30 minutes and get less than an ounce.

What should I do? Any tips and tricks? My husband is watching the baby now and he hasn't eaten for 4 hours. I want to give in and give him the boob but my husband says we shouldn't. The strange thing is he took a bottle of formula three days ago just fine but hasn't since. Same bottle, same formula. He took it from my husband. Thanks in advance for any advice!!