I would like to have another child but really want to be sure it's a boy. How can I do that?

You can't. There is no way to guarantee the sex of your baby.
Same way every time. Urban myth may have you believe you can pre-select your offspring but that is bogus.If you go the test tube baby route you might be able to pretest embryos & have them implanted, but that is tedious & expensive.You get pregnant & get whatever the successful sperm has made.(the sperm determines the sex)you have the same chance for boy or girl any pregnancy.