I'm going to confirm to my boyfriend that he's getting fat
Why would you tell him that he's getting fat?
He has been gaining a lot of weight lately and he needs to do something about it.
Its not the worst thing in the world to be honest about your partner gaining weight. This could be the instance that causes him to want to make the decision to change for the better and loose weight.
It's going to crush his self-esteem, it's bad enough that I'm doing this.
It may hurt him in the sort-term, but if he starts losing weight then he will feel better in the long run.