Unless you work in the health industry, the boss has no business telling you anything about your weight. If you cannot go directly to him and tell him to back off, you need to go to the HR dept. - if you have an HR Department, I'd go to them and explain that he is making a "Hostile" work environment for you (even though this is lunch break, it's still deals with your work environment. ) A note here, since someone has said it.... Going to HR will keep an eye on him and make it difficult for him to fire you for this.
It is also affecting your self esteem, which a boss is supposed to support by being a good boss. (Though to be sure, there are many bosses which have no idea how to be a boss.)