Magen, I wish your father luck and courage. It's great that he has you as his advocate and helper and I admire you for being so caring with him.
I'll let those experienced with it help you with the tongue surgery issue.
As far as a neck dissection, I'm fairly sure we all have different experiences. I'm about 3 1/2 years out from mine and at this point figure I've reached the plateau of how I'll feel from now on. Lingering side effects for me have been the usual chicken neck look and a certain concavity to the left side of my neck as well as numbness under the chin and loss of beard there. In my case too nerve damage has left half of my tongue largely numb.
All of it is a small price to pay for being here and I know you appreciate that too. As others have suggested, even though your dad won't be having any chemo or rads it makes good sense for him to eat as much as he can now. One always loses weight after surgery, especially radical stuff like this.
I think my rather slow return to real food was more a function of the after effects of radiation rather than the actual surgery.
Keep us posted, keep asking questions and I'm very glad you found us!