From what you've written, it sounds like working out will do you a whole lot more good than the boyfriend. But it's easy for us to read a few paraphrased words from your question and assume that your boyfriend is a shallow jerk. Is he really? We don't know your boyfriend or your relationship, so you'll have to trust yourself on this one.

However, I still think it's clear that you should do this. If he still loves the fitter you, great! If his attraction wanes, that's too bad, but you've got to be true to yourself, and if that's not working for him then he wasn't the right guy for you. I agree with /u/Sunjammer0037 that this doesn't necessarily mean he's a terrible person, physical attraction is what it is. It'll be hard, but people change in all sorts of ways, not just physical, and relationships don't always survive the change. It happens, par for the course.

Now... if he actively tries to prevent you from doing something that makes you happy, that you've explained to him is important for you and your body image and self-esteem, that's different. Doubly so if he tries to shame you or otherwise insults you or your body.

I agree with some other comments that said men don't necessarily understand what it means for a woman to lift and develop muscle. They may only know from their own experience, which for most men means bulking. As /u/Pufflehuffy mentions, men often say the same of makeup, not realizing that it can take an armful of products and lots of time and practice to nail down what they perceive as "natural beauty."

Sit down and explain your goals to him, with pictures of your ideal physique to make sure there is no misunderstanding. Ask him to please listen and not to interrupt until you're done. Then tell him you're going to do this for yourself, you're not asking for his permission, and if he can't be supportive then you'd like to know sooner rather than later. I think his reaction will tell you what you need to know.