We will tell…” is actually using the modal verb will/would in a present time sense or a future time sense therefore the present tense /arrives/ is the only choice that matches in grammar. …will tell = We are using our will [right now] to decide what we shall say when he arrives.
Other Possibilities:
1. We will tell him about it after he has arrived.
2. We would have told him about it after he had arrived.
