Loss Of Balance, Unable To Stand Straight, Arthritis, Nerve Issues. What Seems Wrong?

i notice that when my boyfriend is standing up straight his left side of his body seem to tilt sideways and at times when he takes a few steps or standing still, it is as though he takes a few quick little side steps sideways also and it is as if he had to catch his balance or he could fall. at times his posture is in a straight upright position but i notice that as the days go by he is in this side bent position, when i tell him he is tilting to one side he straightens up to normal position but i can still notice the tilt in his body, he has no previous history of medical problems, i know his legs bother him in the morning but the doctor took test and said he could have a little arthritis in his legs as they are always stiff in the morning, he is 77 years old, he exercises 3 times per week does not smoke and eats clean i do not know anything about medicine, but i was wondering if this could be a nerve problem or a muscular problem youre advice would be appreciated my name is Diane Brown