I'm sorry this has happened to Finn and know you will be very concerned for him, especially given all of the vomiting.
It is important we clean the area that was stung and try to remove any stinger.
The main risk here is that he may develop dehydration and low blood sugar, as jellyfish stings can cause quite severe vomiting.
Depending on the jellyfish, we may see more serious signs such as trouble breathing, abnormal heart rate and blistered skin.
It is always best to see a vet if possible, to examine Finn and the area that was stung.
He likely would benefit from some anti nausea medicine, antacids and fluids at a minimum.
I appreciate it is late, but there should be an open clinic nearby.
If unsure where this is, please send on your postcode.