Re: how do I stop being so selfish
On the other hand, if his father is dying and he's providing the family support one day a week and the "reunions" are meetings to work out estate issues and make joint decisions about his continuing care, then you have yourself what's called a "keeper". (Trust me, my siblings went as far in the other direction as they could.)
And if you have met the Dad and know that his situation is true, then I think he needs and deserves a visit to the gym by himself. But I've been there, done that. It sounds like it won't be all that long before the situation resolves itself with the Dad's death. After a window of time, then I think it would be okay to approach the idea of working out together but if the father's illness is "real", then he deserves his space right now.