Hi there,

Having worked in an animation studio, I can tell you that production is on-going till the release of the film. However, the talent recording sessions are separate in this process, in that, the recording sessions are usually done off-site, usually to wherever the actor is at the moment. Studios like Dreamworks Animations will not pass on fanmail to the actors (at least that I am aware of).

Your best bet would be wait till he's on a live-action movie and send via venue or to the listed fanmail address.

Hope this helps!