It definitely seems genetic. My husband produced one autistic son and then another with mild OCD and both kids have pretty problematic social disorders.

Anxiety disorders run in my husbands family, but BM's family is a nightmare. Super white trash with extremely low intelligence all around. I can deal with the special needs aspects of my skids, but coming to terms with the fact that they have pretty low I.Q.s and that I can't change that has been hard for me. I grew up in a family of prodigies (music, dance and writing) and was surrounded by really smart sisters and raised in an academic environment where a bachelor's degree was considered as essential as a High School diploma. My mom was a college teacher and I grew up on university campuses and spent hours and hours sitting in the back of classes. Getting straight A's, being proficient in at least one art form and language and generally being high achievers were basically required in my family. I have always loved reading and books and partially because of genetics and partially because of my environment, I grew up with high expectations and the feeling that my family's oddly high academic standards were normal and that all kids were highly intelligent and talented and it took being an adult for me to see that my family was pretty unusual. I have a lot of smart friends who don't like reading and grew up in average, normal families, but I also have a few friends who grew up in families like mine and they tend to be like mine, in that all of the kids from the family are very accomplished and academic and tend to follow in their parent's footsteps (becoming authors, teachers, travelers, musicians, dancers, writers etc...) and then do the same with their kids. Family environment has a huge impact, but families tend to be filled with siblings who tend to have the same levels of intelligence and similar genetic problems (like anxiety disorders or autism). When trying to figure out what someone's kids will be like, look at their family! I think it's the best and most accurate way to assess if someone is likely to have stupid or smart kids for example. My husband is very smart and comes from a wonderful family that is pretty average for an upper middle class family. All of his family members have had professional careers and been well educated, but were never academic-obsessed or super literary or anything like that. BM comes from pure white trash and her mom dropped out of high school when she got pregnant (on purpose at age 16). BM graduated from High School, but not college and is shockingly stupid. I definitely see the mix in my skids and it's unfortunate that they have 50% genetics from incredibly stupid, white trash loser genetic pool and 50% my H's side, which is great apart from the anxiety disorders.

If this is what you are looking for, I would recommend talking to a doctor about concerns for genetic probs (like inherited autism) and also do an informal scan of his family and his kids family. If you have a special needs step-child and you see that BMs side of the family has tons of problems, it's a small (if unpopular) leap to come to the conclusion that her side is likely to be the culprit of genetic disorders, but it might be your husband's side too. Anxiety disorders and cancer run in my family and it was part of my decision to have a hysterectomy. I didn't want to risk passing on cancer, endometriosis and anxiety to my children. Fate wound up handing me two sons with severe emotional problems, but they didn't come from my body, thank God!!!

Look at family and genetic history for your first clues and talk to a doc if you're worried about any future kids with your man.

But there is no Pre-natal test for autism. And even when there is (maybe one day) there's such a wide scale of affectedness.

If he has a child with autism there's no real way to predict if it will happen again. I haven't done any research so a geneticist maybe be able to break down the probability of genetic reoccurrence.   
Life is a gamble. There are a million other things that can go wrong during pregnancy not to mention after.    If you want kids, go for it! I wouldn't fixate on this.   There is a lot of early testing available now.  
 

My only concern would be how much work the current special needs child is and will be when consedering an addition.