He'll be just fine!
We're going to Vegas one month from yesterday (YIKES!) and my son (my youngest of 4 and the only boy...totally a mama's boy) will turn 6 months while we're gone. We'll be gone for 4.5 days. He has some issues with the bottle because he's just not use to it. I'm a SAHM and there is usually no need for him to take a bottle. I don't go many places that he can't go with me. I started right after we booked the trip (about 2 months ago) pumping like crazy to amp up my milk supply. As a result, I now have about 125 oz of milk in the freezer (my goal is 150), and my milk is letting down about every 1.5 hours, lol. I have just eased my own fears by knowing that I am leaving plenty of milk for him, even if he chooses to not eat solids while I'm gone...its still touch and go with the solids. I also know that no matter how much he protests the bottle, he will not starve himself, nor will Grandma allow him to starve. He'll eat when he's hungry, and they'll (him and grandma) will figure it out. I NEED this time with my husband, and I know it will be beneficial to everyone...especially my 4 kids when I come back rested and refreshed.
This trip is a HUGE leap for me. Last summer we left our (then) 14 month old daughter for 3 days while we took the older girls to Kings Island and I was a blubbering mess for about half of the 4 hour drive there, not to mention the weeks leading up to it, lol. Turns out, Grandma did just fine, and everyone survived!