Why doesn't my boyfriend/child's father understand that I want to get breast implants?
You can't MAKE him understand. You simply tell him you intend to get implants, if that's what you really want. Although, it's a VERY unhealthy mindset to feel you are less of a woman because you have small breasts (the amount of fat around your mammary glands doesn't dictate how much of a woman you are). I've had 2 friends who got implants. One because she had very small breasts and the other because hers were 2 very different sizes. BOTH those women were very happy after getting implants and wish they'd done it sooner (both were in their mid to late 30s when they had it done). Research all the cons so you'll know what you might expect, and then do as you please. It's your body and you need to be happy with it.
I don't think you get it either. He likes what he likes, and that doesn't include implants. If you really want them, get them. But be reminded that if you decide to have more children in the future, the implants, perhaps, should wait.
I can see where you are him are coming from. I believe you are upset because your chest (which is often representative of womanhood) is no longer what is used to be and you feel sad because it makes you feel less womanlike. However for him, he probably sees your chest as a tool for you to attract members of the opposite sex such as him and other males. You just have to make him understand that he care for him as much as he already cares for him (evident by his unconditional acceptance of you, which I find quite sweet) since he seems to be insecure. Though the contention still comes with what to do about it. It will probably make him feel more at ease if you learn to love yourself and dismiss the idea of your chest being representative of your femininity. However, if you yield to your self-consciousness and do something like get plastic surgery, that is going to create A LOT more problems than for what it solves. Good luck!
Grow up and get over it
As long as you can pay for it yourself, do what you want - as long as you are supporting yourself and your child, paying all your bills on time, and saving for your child's college education (you should be saving approximately $300/month for college - for the next 16 years!). If you are asking HIM to pay for it, that's not fair. If you're going into debt to pay for it, that's stupid. If you're using money you should be saving for/spending on your child, that's irresponsible.
Every time you see signs of getting older as the result of age or giving birth you can't run to the plastic surgeon every-time to fix what is inevitable. There wouldn't be anything that would convince me if my wife wanted to mutilate herself like that (cutting holes in your chest to jam plastic bags in is just that). Put the money in your child's college fund instead.