Sorry to hear about the recurrance so soon after treatment. I've had three oral primaries and have had good quality of life in between. I thought I had it beaten four years after my second surgery. It did come back just after that but it might not have and there's always the hope that no further primary will occur. I think there's a lot of hope for your father and wish you both well.

1996, ovarian cancer surgery + cisplatin and taxol.
September, 2007, SCC of left lateral tongue. Excision.
October, 2009 recurrence in scar tissue, T1NOMO. Free flap surgery from left wrist - neck dissection. 63 year old New Zealander. No chemo, no RT.
February, 2014. New primary in left buccal mucosa. Marginal mandibulectomy, neck dissection, right arm free forearm flap. T1N0M0 but third occurrence and some areas of concern: RT started 8 April and finished 19 May.