I'm ashamed of my boyfriend for being fat
Why does his weight make you ashamed?
I don't like the way he looks and I feel like no one else will like him
If you're ashamed of your partner for his weight then you should speak with him about it and work out a plan to where you're both happy. If you want to be with someone, then you judge them so poorly, and he deserves to know how you feel. If you remain ashamed, then you'll both grow to resent each other and he deserves someone who loves him for what he looks like.
I'll change him. It's only fat. I'll make him look like a unicorn or something.
I hope that you are kidding about wanting to make him look like a unicorn, and I should also warn that it is difficult to drastically change someone's appearance. Many people are born with a natural body shape, and there is little that they can do. Concentrate on helping your boyfriend be healthy, and love him for who he is.