How do i get my boyfriend to exercise more with me?

My boyfriend and I both could use more exercise but its vrey hard to motivate him sometimes.... yes we play tennis sometimes but thats only when he feels like it. I have tried to get him to walk with me in the evenings, go biking, anything really, but to no avail.... He keeps saying that he wants to but its all talk and no action..... what can i do to get him motivated???