I'm not entirely sure; I'm also not sure if there has been any research into the exact chances of a couple, one with AS and the other NT, having a child on the spectrum. I can tell you this: my father is an undiagnosed Aspie, my mother is very firmly NT, my sister is also NT though I am an Aspie as well; my dad's sister on the other hand, does not have AS, nor does her husband (though AS runs in my dad's family), and only 1 of their 3 children has AS.
He is not alone in worrying about passing on AS to any children he may have in the future; I believe that all Aspies considering having children worry about it. You are right, a parent with AS (who knows about it) will have more insight into their child's (if they too have AS) mind. It might be a nice thing to remind him that, if the two of you do decide to have children, as an Aspie himself, he would be more likely to notice the signs of AS at an early age so any appropriate steps to get help or a diagnosis can be taken.
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Dance like no one is watching. Sing like no one is listening. Love like you've never been hurt and live like it's heaven on Earth. -Mark Twain
If life gives you lemons, make grape juice, sit back and watch the world wonder how you did it.