I would try to press the vet to prescribe pain relief, especially if the vet himself has acknowledged that he is suffering. Dosing guinea pigs is simpler than most vets seem to think, it only involves reducing the cat/dog dose to the guinea pigs weight then increasing by the species' metabolic rate. (Okay, sounds complex like that but it's just two basic steps). For most piggies, the medicines and doses suitable include:
Rimadyl (Carprofen) - 0.2ml injected OR a 5mg or 10mg tablet
Metacam (Meloxicam) - 0.1 - 0.3ml oral liquid
Children/infant ibuprofen suspensions can also be given, usually at the dose of 0.2ml orally. In UK, Chidren's Nurofen or Calpol (which contains paracetamol instead of ibuprofen) can be given.
I am not sure what is available in Australia (I think I read you live there?), but especially with Boris' condition I do think your vet ought to prescribe an animal pain-reliever, at least for the first week that his splint is in place. Definitely something to discuss with your vet, it may make all the difference.
Boris needs to eat normal hard foods - hay and dry feed - to keep his teeth in check; feeding soft foods only will allow the teeth (molars in particular) to overgrow and you will be faced with a whole other manner of problems which you may have difficulties treating with his spinal problem and splint.
I do agree that keeping him in a limited cage space is important, but perhaps position the cage next to his friends so he has some interaction with them, and work out a way to lift him (perhaps encourage him to step onto/into a hard surface or enclosure) so you can let him out for regular cuddles - preferably at least 3 times a day.
You are doing the right thing by questioning and aiming to improve his quality of life while he is in the splint, as piggies do become depressed if they are not allowed normal routine or natural behaviours. I'm sure, with lots of TLC, good vet treatment, and a gentle understanding, he will learn to cope with his condition and it's treatment.