Assure your father that it's very common for the neck to swell and for there to be strange feelings on the side of the neck of the affected lymph node. I finished 39 IMRT and cisplatin Feb 14 and weeks after that was awakening each morning to a flabby neck and sometimes swelling 1 or 2 inches under my left LN along with discomfort in the area. Each morning is different and this morning for the first time the swelling under my chin was less flabby and more hard like. Swelling is usually gone by noon.
My radiologist has told me that this is normal draining of the lymph system. For me, other than this morning, it's been less a problem in recent weeks.
And to put this in context, my March CT scan showed everything looking good and the LN reduced greatly from pre-treatment size. AND the April CT scan showed LN shrinkage, compared to March, of 26%.
I hope my experience gives your father comfort that it's probably nothing to worry about. But, he should have his radiologist take a look ASAP.
Wish your father the best of luck! Steve