Oh my, this is a touchy subject. Although it is likely a common issue, with the rate of divorce and separation and the number of children who are diagnosed with ASDs. Ideally, it would be great if all adults involved could work in favor of the best interest of the child(ren) involved.
It is certainly optimal to have a child receiving appropriate services (speech, occupational therapy) if he does, in fact, have developmental delays or disabilities. Just recently, a friend of mine mentioned a boy who exhibits many "aspie"traits, and his mother reacted in much the same manner when it was suggested that her son may have Asperger's. Some parents don't want to face the possibility of their child having "a diagnosis", and it is easier for them to not address it. Since dad has mentioned that he suspects there is something going on with his son, hopefully he will take the initiative to have him formally evaluated so the little guy can be eligible for services in their area.
While I agree that parenting issues can get very ugly very quickly when mom and new girlfriend disagree on anything related to the child, new girlfriend is becoming a part of the family and it is wrong to ignore the possibility that the boy has autism (or something) simply because mom doesn't want to deal with it.
The only advice I could offer is to keep things respectful, and keep the focus where it should be. Since dad has already discussed this, I don't think it is unreasonable to simply agree, note similarities that you have seen, and let dad know that early intervention does amazing things - especially with communication/speech.
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Mom to two amazing children diagnosed with autism: 7 year-old daughter and 5 year-old son.