Going On Holiday While Breastfeeding – How To Ensure Milk Supply Doesn’t Decrease?

My baby is four months old and I’ve exclusively breastfed him so far.
I’m going for a long weekend away next month (he’s staying at home with his grandmother).
I’ve been using a breast-pump, and built up a hefty supply to leave for him.
I have two questions really;
1. Although I’ve stored up a lot of milk, I feel like there might not be quite enough. Should I leave emergency formula in case it runs out (I’d rather not use it at all if possible), or would it be better to ask my mother to alternate formula/breastmilk feeds to make it easier for him to deal with?
2. How long does it take for breastmilk supply to drop? I intend to take a pump with me so that I can use it maybe 2 or 3 times a day – how likely is it that I’ll get home and find that my body is producing substantially less milk? If so, any suggestions on what I can do?

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