Okay I looked back and reread part of this (so forgive me if I missed any information) - You're dad's cancer will likely not be cured by chemo. If there is NO REASON why he can't be operated on (the fact that he was already radiated and this can cause issues with healing - might be one of them), surgery should be the preferred form of treatment. Since it has not metastasized, it is a good thing to get it now before it does. Unless the surgery will strip away his quality of life, then I would push for that. I must say 2.5 x4.5 is a large tumor... not huge but big enough. Mine was 1.5 x 2.5 and that cost me a third of my tongue. A tumor that size will likely not respond to chemo. Of if it does, it will only be minimal.
I am not sure of the details of your dad's tumor and the structures around it that may be effected, but really, unless there is really good reason not to operate (again the radiation he's had may be an issue with healing - but a few people here have had salvage surgery after rads and they are okay), then as I said, surgery should be his treatment. I can't say anything for certain of course as I am not a dr. but from what I know about this type of cancer, the chemo is palliative not curative. Someone else will correct me if I am wrong - I am sure - but this that's pretty much common knowledge around here.
best of luck... PS tongue cancer can be very painful, this is normal.