How to clear catarrh in nearly 4 year old?

[BlackOrangeFrog]

Da has had what appears to be catarrh for weeks.
Pharmacist won't give anything, doctor suggested antihistamines, to no effect.
I've tried olbas oil, nothing m
NHS says you can't give decongestants to under 6...

Will be taking back Dr hopefully in Monday
But any advice?

Sorry, he's nearly 5 (December) 🤦‍♀️

Test this yourself to make sure it's not too hot; but traditionally people used to put a towel over their head and inhale steam from hot water.

We've tried that, steaming water and olbas oil, made a little den! But no joy