I am old fashioned in that I don't believe in calling a guy unless you are seriously dating. But in this case, maybe you could just call him back to explain why you missed his call. After all, he did make the first move by calling you.
Calling you is probably difficult because he risks rejection and maybe he is putting it off. He did try though so maybe you should give him a break and call him back to say you're sorry you missed his call.
That doesn't seem too forward to me.