Ultimately, you're going to need to either donate or toss some of it.
First, hopefully you're rotating your stock (i.e., sometimes freezing all of it and giving him during the day frozen older milk). Milk in the freezer lasts for a fairly long time, but not indefinitely; Medela for example recommends 6-12 months, and that assumes you followed good practices.
You should also verify from time to time that frozen milk is still acceptable to your child. Various reasons - particularly, lipase levels, poor storage practices, freezer defrost cycles, for example - can cause it to go 'off' faster; and sometimes your child is just more sensitive. Make sure he's still happy to eat your milk frozen - otherwise there's not much value to keeping it in the freezer, is there?
Second, the most likely reason you might need a significant storage quantity is if you get sick. If you catch a sinus infection, for example, both the infection itself and the medicines you may need to take to treat it may reduce your supply - or you may wish to take medicines you'd prefer him not ingest, and then not feed him at all (just pumping and tossing) for the week or so it takes to treat the infection. (Talk to your doctor before taking any medicine that's not approved for breastfeeding mothers, even if you choose to toss the milk - some medicines may have metabolites that last longer than you think.) In any event, that's when you're most likely to need the excess; so I would keep enough for a full week's feeding at least. How much more than that is up to your preference and your storage abilities.
Third, you may wish to think about whether you're likely to take a trip without him - either for work or vacation - which would necessitate using stored milk for an extended period of time while his caretaker (dad, grandma/grandpa, etc.) feeds him from the frozen supply. If you travel regularly, you may need a higher stash than normal to avoid needing to ship milk back (which can get expensive).
But either way, make sure you've got usable milk - it would be very frustrating to finally need it, and find that your son doesn't like it or that you have a higher lipase level than normal and the milk goes off quickly.