Potions are a way of storing an effect. The effect is chosen when the potion is created and can not be changed or directed afterwards. At least that is the way I understand it. That said, what you are throwing is an object, not a spell.
If you want to treat it as an evocation, I would make it a rote spell with a focus component which would be the potions. Instead of a constant focus like a staff he would have to create his foci over and over again. You could have him only carry 4 or 5 of those focus potions around, but instead of the usual focus slot rule, they would have one or two additional slots without cost (and I would restrict those to +Power only), because they are very limited foci. Recreating the potions would mostly be colour, because a wizard is considered to be able to maintain his foci. However, you could have him perform a ritual, gather herbs, maybe even "compel" his focus dependency, having him run out of resources just as he is about to enter the final showdown.