General

I would take it slow.

You can start out by saying you won't diet anymore (if you were a BBW to start), or you won't diet. You can say you enjoy food, etc.

Then you can say you like it when people admire your weight, and that some people admire your weight gain.

Sooner or later, that person will figure it out, or you can tell him without everything being a complete surprise. You can also stop saying things if it does not go well.

If this friend is truly a friend you should tell him without any problem.Personally when I said this to my friends I was anxious about their reaction but after telling they said to me :was you worried to tell this to us? C'mon it is your life and if you are happy also we are happy with you.