I can't help but agree with metalbudderly. Your bf should not be pressuring you or making you feel unhappy about your body. So even if you do decide to do something to your breasts, it should not be becuase of pressure from anyone else.
To be honest there's little you can do to naturally increase breast size.Sometimes if you're very skinny your boobs can naturally also appear to be quite flat, so sometimes when you eat more, your breats also appear to grow in size.
There's always padded bras or different clothing that you could opt for but i wouldn't recommend surgery either.