More likely he will lose none at all.
Hmm, I'm sure he'll lose one eventually. Norrel (Slapsticks) and Largan (Addridae) are still growing, so they'll be able to contribute to the war effort in a little bit (unless they find their own war between now and then). I'm about to stop growing once I get the remaining planets around my home planet (only a couple more planets need arcs sending to them), so I'll be pumping a lot more into aggressive ships.
So short term, he might not lose a planet unless he really messes up. But longer term, I think he's really going to suffer for not expanding a little. We just need to keep a lid on him, try and make sure he doesn't sneak anything past us.