The decision is up to you of course - the surgery may be trying depending on his health if he is willing and able to do it -
Then it's actually not too bad of a recovery assuming the surgery goes okay and he doesn't have problems healing . (Diabetes etc.)
Follow up treatment for this is normally rads if there's nodal involvement however if your surgeon does the surgery (and removes the nodes - and feels he's gotten it all) then that may be all he needs. Is your dad a young 90 in decent health? Or is he more complicated ? This and his input should be what is used to determine what should be done - for what it's worth the surgery isn't that bad it looks dad worse than it feels. Take care