M.P.
Because the medical community does not know the cause of autism I suggest that, as every mother knows, you can't know ahead of time how your baby will be. Because there are genetic links to many conditions and possibly to autism, I'd have a genetic study done before making a decision.
As Brandy M. says, children are a blessing. My grandson is on the Autism Spectrum and we wouldn't have not had him in our lives. He's a challenge but he's also loving and cheerful. A part of your decision should be based on your resources. How does your boyfriend relate to his children? Is he able to deal in a reasonable manner with the difficulties? Are he and his ex able to work together for the good of the children? What sort of stress do his two children put on him and is he able to manage his life in a calm and positive way? If having another autistic child would be too stressful for either one of you, don't take the chance.
Are both of you mature enough to put children first? Do you work together and give each other support even in stressful times? Are you financially secure? Are you able to establish a secure home before having a baby? Have you working thru your own emotional issues to the point that you feel confident in ;yourself as a person? Ask yourself all the sort of questions any couple should be able to answer in the positive before having children.
There are some conditions that can be predicted based on genes. There are some that have greater odds of happening a second time or because of age or current health and environmental conditions. Autism is still an unknown. It is very important for prospective parents to consider whether or not they can manage if the child is born with some difficulties, including the normal ones of being a baby.