I don't think I would recommend a bike right away. Based on his current level of fitness I don't think he would enjoy it very much.

Before beginning any exercise plan, he should be seen by a physician.

I would start with walking. A short walk in the morning or the evening is an enjoyable way to begin the process of becoming more fit. He can walk as much or as little as is comfortable and gradually increase the amount of time spent walking over a few weeks or months. This approach allows him to begin with little equipment or expense and build on success. It also helps build the habit of exercising. After a few months of these regular walks he should feel much more fit and much differently about exercise in general.

If he is still enthusiastic about fitness after a few months of walking, then he should look at cycling.

Dan