Actually, 55 years old is still a good age, the bones still have considerable strength, you can carry out some instrumental exercises, because your father sports less, do sports can still have a significant physical recovery process. Dumbbells of the right weight are a good choice, dumbbells can do a lot of action, but avoid lifting dumbbells above your head, half squats, half push-ups with knees on the ground are also very suitable exercises.
It should be emphasized that middle-aged and elderly people have inertia, since they want to exercise, children should be supervised, so that it becomes a habit, and pay attention to its safety, it is best that you and your father to exercise together.