First let me say good on you for including your father. What weather conditions are you expecting? If it's warm enough for summer weight gear that would cut the weight some but don't scrimp on his sleeping pad. Us old guys need a soft bed. "Occasional back problems" seem go hand in hand with being over 50. Trekking poles help me a lot on that account. Also a pack that moves most of the weight to the hip belt is a big help. Keep an eye on him during the hike. Male ego being what it is he may not tell you if he's burring out.