D.K.
I worked full time shift work with my oldest and exclusively breastfed (my command had a lactation room and hospital grade pumps so all the nursing mamas had to do was buy their own attachments). At about 8 months my son started a growth spurt so he was taking 6 to 8 ounce bottles by that time. I would say that for three days you should put aside 5 bottles per day of about 6 or 8oz each (you'll have to decide which, and you can experiment before you leave to see which seems best, start with 6 and move up if need be) and then set aside an extra bottle worth per day just in case of accidental waste or something. That would work out to a total of 18 bottles.
Make sure to have whoever is watching him thaw the milk in the fridge. Every night they should put the milk set aside for the next day into the fridge to thaw. That way, it is thawed by morning plus they can try smaller amounts with him first this way without wasting any of it.
Hope this helps and have a good time on your vacation!