A few things-
1. Check low and high. I've heard of babies getting into light fixtures and the top of drapery before and into tiny cracks in the wall.
2. Put a clear bowl of wiggly worms out with flour around it. If he comes to check out the worms, he will leave little footprints and you can trace him.
3. If you can crate your dogs or keep them out til you find him, that'll be best. I have two dogs so I know it's not easy to keep them out, but they'll be okay.
4. Turn up the heat or turn off the AC to make your house hot so he'll be more active.
Keep us posted. Hoping for the best,
Sara