Your dad has undergone major surgery and will have quite a long recovery period. I'm not sure how many weeks it is since his op but he should be expecting about a 3 month recovery. Even after keyhole surgery my husband was still tired at 3-4 weeks. now 6 weeks on he is much more back to normal. His sleep at this stage might also be disturbed if he can't get comfortable which won't help.
Sometimes I think Dad's expect too much. Mine is also 67 and very active, he had keyhole surgery to repair a hernia yesterday and despite his protestations about getting on with things he was very gingerly getting into the car when I picked him up from hospital last night.
All that being said if he doesn't feel right it might be worth a quick trip to see his GP to just get the once over, BP, Iron levels, etc.