I don’t think it’s immoral, and I also don’t think it’s immoral to have the child. It’s more complicated than “disability” or “ability”, if you can handle the job and give that kid the best life they could have had, short or long, love them and see it through, that is not immoral. If you know you cannot, and it would wreck your life or be very detrimental to your already born kids, it’s certainly not immoral to abort the fetus and focus on what you can do.