[Hotfoot]

Work up to communications mimic, then screw over all of his alliances. If he is an uber-newb, he's going to be ultra-reliant on trade with his allies. You would seriously hamper his economy, and thus buy yourself some vital breathing room.

Also, once you can get into his space, consider sending some plague bomb ships (with as much ECM/stealth&scattering armor/ship&fleet experience as you can muster) in to his colony worlds. Go for the ones least likely to be defended and pick them off one at a time. After that, try to plague his major colonies. Then follow up with your primary invasion force.

That should be enough to get you started.