I am exclusively breastfeeding my 4.5 month old...

I am exclusively breastfeeding my 4.5 month old but have a function to attend in a couple of weeks which will require me to be away from my boy for several hours. I have tried to give him a bottle of expressed milk every day for a week but he totally refuses to take it and seems traumatised afterward by screaming for hours! How can i get him to take the bottle of expressed milk from somebody he doesn't know if he won't even take it from me??