What goes on in the house when your DH gets home, Sophie? If the lights are all on and you and your DH are catching up noisily before going to bed, that could be enough to confuse your DS into thinking that it's 'awake' time...? I'm just digging around for ideas
I thoroughly agree with Naomi - keeping it dark at night and light in the day was the key for us with both DD's. We didn't put curtains up in DD's room until she was around 10mo and she started waking up waaaaaaay too early, LOL. Maybe you could gently suggest to your DH to tiptoe in at night and try not to use many lights?