I agree you need to ask why he said it. It could be that he has experience of the autistic spectrum and recognises traits, and knows that with additional support, your DS will be all the fab things he already is, but better able to socialise and handle change/different expectations, and a happier person. Or it could be that his kids are poorly boundaried and rather than admit that, he would rather slap a highly stigmatised label on your son instead. One is a lovely thing, and actually very brave. The other is ugly in the extreme.
Does your DS have a lot of fussiness around food? Does he notice smells, sounds, tastes a lot? Is noise a real problem for him, or do clothes often feel uncomfy? Those are also often symptoms. As are special interests to the point of obsession - it's been joked that NASA is a community workshop for people with high functioning autism and John Elder Robison left school at 17 to go on tour with KISS, customising their guitars, because his Aspergers meant he was obsessed by them and therefore uniquely expert! Which is, gently, to say high functioning autism - if he has it at all - need not be all bad. It can be harder, as it goes hand in hand with higher levels of anxiety than normal, but it can also be enriching, when you have a child who asks extraordinarily demanding and complex questions from a young age, soaks up info, and will concentrate on something that interests him for literally hours. It doesn't mean he has to be obviously brighter at a younger age at all. It can mean his powers of concentration on something that happens to interest him are unusually great. Could be anything - Pokemon, Star Wars, Lego. It's that focus that is diagnostically important.
DS has ASD. He's also been described by every teacher he's ever had, and his diagnosing paediatrician, as "delightful". His "lovely temperament" was actually cited as a developmental strength. He's loving, kind, and a complete joy. He has friends, he just needs help sometimes - explicit, spelled out explicatory help - in social communication and interaction, and we need to be aware of his sensory sensitivities. But otherwise, he's fine. The stigma around autism disguises that he tastes and smells better than you or I, sensations are stronger and sounds too, and while that can be bad, think of how many situations where those super-sensitivities are benefits? He's also immensely bright because all intelligence really is, in my opinion, is the fascination with information and the ability to find it and retain it. And someone with a condition which means you become obsessed with certain things has a head start there. All in all, I feel lucky to be his mum, and his paed agreed with me that we hit the jackpot with him.
If your DP has high functioning autism in the family, he may not see it as a disaster. If I see aspects of my son in other kids, I'd see it as a good thing. And given mothers always adore him, he has stacks of friends, and most people never know he's autistic at all because he does not fit the stereotype, I don't think that's unreasonable. Autism is not necessarily and always disability. It can be fascinating because autistic children think in startlingly original ways, because they aren't fully able to conform. They are free thinkers in a very real sense.
So I'd take a deep breath, ask DP why he thinks that, and then evaluate what his statement means. Because if he does not know anyone on the spectrum, and he's insulting your child by labelling him with something so hugely demonised in mainstream public opinion, he's actually saying he resents him, that he resents how he shows his kids up, and wants to think there's something abnormal about him. That is not pretty. On the other hand, if he;s worked with, or related to, people on the spectrum... he may just think your child could benefit from some support. Talk to him.