Does Ezreal, Prodigy reduce the additional repeat cost created by Temporal Portal?

Yes. Repeat granted by Temporal Portal is still the Repeat keyword, and Repeat is an optional additional cost, so Ezreal, Prodigy's discount applies to it (FAQ #8187; Rule 820.1).

Temporal Portal's Repeat cost equals the spell's printed cost (not any discounted amount you actually pay). When you choose to pay it, that additional-cost component is added to the total, and Ezreal's discount reduces it by [1] or [A] right then (Rule 356.4.c). So a 4-cost spell's Repeat cost of 4 would cost 3 Energy if Ezreal is on your side. Each granted Repeat instance can still only be paid once per play.