Hi , I'm James DVM MRCVS and would like to help you with your question
Sorry to hear about Oscar
Has you vet seen the lumps in his mouth ?
what is the state of his teeth ?
It hard to say what is the cause for the lumps in his mouth - is it an infection , is it a reaction to some irritant material that Oscar could have had , Is it a tumor that has spread through his tongue. ll these need to be investigated an further more if Oscar is not eating anything since he is so painful , then he needs stronger pain relief and even considered to be hospitalized for fluid therapy and administration of pain relief which you can not give at home but which will make him feel much better.
It regards to checking up on his tongue then the following needs to be done :
1. Blood test the check his general condition , see his level of hydration . organ function and the level of his white blood cells (will help us see if there is an infection brewing).
2. Chest and abdominal Xray (perhaps an ultrasound as well) to see the chest is clear of residual tumors (metastasis) as well as have a view of the organs
3. Starting on Antibiotics as soon as possible and examining the teeth which might lead to a localized infection as well .
Right now he is getting on Tramadol which does not seem to be helping greatly and he seems miserable , So my advise is to seek your out of hour vets help , even if they can no conduct all these tests (which I believe they can) , they can keep Oscar with them for further treatment and most importantly pain relief and antibiotics - this will make Oscar much happier already and will as well allow you vet to maybe take a sample form these lumps on his tongue and have analyzed by a lab (if have not improved by the use of Antibiotics)
I hope this helps.
If you have any more need for my help , please let me know.
All the best