If your son really feels that this boy is worth maintaining a relationship with, ask your son to invite him to your house for dinner. If he declines, ask your son to attempt to speak with this boy alone and to tell him that this teasing hurts his feelings. Tell your son to ask this boy, "Why does it make you feel good to make me feel so bad? I would never do anything to hurt your feelings or embarrass you in front of other kids. Are you my friend? If you are, then please stop calling me names and making fun of my weight. Can you do that?"
If the boy ignores your son's requests and continues to tease him, your son needs to seek out the companionship of kids who aren't associated with this boy. He may also be able to stop this boy by responding to the teasing. He could say, "You know, I do weigh more than other kids and some kids have big noses and some are really short. So what? Does that mean that you're better than all of us? Why do you need to make fun of other people, anyway? Can't you find another way to feel good?"
You should monitor this situation closely. When kids are teased often, they can become quite depressed and may feel like they are helpless to stop those who harass them. Don't be afraid to step in and seek a meeting with this boy's parents if all other methods fail to bring a good result.
I'd also ask your son if he would like your help in becoming more fit. Maybe there is a male family member, school coach, or friend of the family who can get him started on a fitness program that involves aerobic and weight-training activity. You also need to consider how the types of food he eats might be causing him to become heavier than he need be.