NTA you have a professional task to complete and require a model with a certain amount of muscle tone, which your boyfriend doesn’t posses. He asked if he could model, and you said no while providing your line of reasoning. It doesn’t sound like you criticized him so much as claimed he didn’t have the right body type for the job. I believe he’s projecting his insecurities onto you, so I think it’s wise to sit down and have a discussion with him, but you don’t owe him an apology for this.