Do phone 999, this is a potential emergency.
Do not hit him on the back when he's coughing/retching, as long as his colour remains good/he's clearly still breathing OK. The coin is NOT blocking his airway but might move to block it if you hit his back/give him the heinlich (sp?) manoeuvre. Do keep him very still (for the same reason)
Do tell him that you will be going to hospital with him (ds2 thought we'd just let some strangers take him away.....bless)
Do tell him that the Doctors won't be cutting his tummy open if he swallows the coin into his tummy. You can go and get his tummy metal detected (coin will do no harm once it's past the neck) so that you can all get a good night's sleep.
Do warn ds1 that the coin that he got from the tooth fairy that ds2 has just swallowed will NOT be dug out from ds2's poo in the toilet and he'll just have to make do with another £1 from mummy's purse.
Remember to laugh about this afterwards and make mental note never to leave any coins anywhere that money obsessed 4 year old can put his hands on.
(PS - work out how to explain to school caretaker that ds2 has seen the coin NOT flush down the toilet when he did a poo at school a few days later...)