Hi, if I am not mistaken, you dad must have had radiotherapy as part of treatment. Take him to a doctor, it could also be late side effects.

Father; 67 yrs; RIP: 2012/05/26

TX:SCC pT2N1M0G2;Glossectomy+SND+CCRT(59.3Gy+6xCis.)[2009]
TX:Nodal Mets; 3xDCF[2011/05/05]
TX: RND + PMMC Flap[2011/07/11]
DX:SCC PNI+ECE
TX:Re-RT 60Gy[2011/09/21]
TX:Gefitinib 250mg[2011/12/18]