Have a look at his abdomen under his ribs, if it's slightly sunken then he's not breathing quite right - don't freak he might just need a different spacer. Any signs of trouble breathing of course take him straight to emergency.
Do you have a vaporiser in his room? As the air gets colder and dries out in terms of humidity it can make coughs worse.
Also have you tried Vicks on his feet with socks on at bed time? I know it's an old wives tale but it worked a charm on my two.