The words Bio/birth Dad, and everyday Dad are useful in this conversation.
His dad, your husband, *really* is his dad. That's his Every-day-daddy, and he also has a biological father, someone who helped make him, but who wasn't capable of helping to raise him.
My suggestion is to start doing some reading with him NOW. Get out a series of books on the different ways different families look. Some people have one mama and one papa, some people live with their mama, some people live with their papa, some people live with two mamas, or two papas, some with their grandmother, some (like in my house) live with their Auntie, some people live with multiple generations, some have siblings, others don't, some have an everyday dad and a bio dad, some have an everyday dad and a step dad, and so on and so forth.
You know what is best for your family and it's important that you feel good about the way you bring this up.
My suggestion is to start introducing concepts of different family structure *before* you have start a series of discussions that introduce his own family of origin history.
While it is a lot for a child to digest (learning that their history is different than they've assumed/been taught), there is no shame in different family structures.
Also, don't hesitate to have a few professionals in his corner. You could speak with a child psychologist or two, and talk it out with them *before* you begin these discussions. Should you need help answering questions, or accessing support for him, they would already have a relationship with you and your family and could help him ease his way into the knew knowledge.
Our culture puts a lot of emphasis and importance in the "typical" (actually not as typical as we think ;-) family structure: one mom, one dad, and the kids all living under one roof. For those of us who don't fit in to that template, it does create some friction, simply because the "outside" doesn't treat our family structures as being as tight and as valuable. The books we read, the movies we watch, our cultural language (real dad vs. step dad), etc. emphasis this idea.
In fact, our relationships are just what they've always been. They're names may change, but our love and memories do not.
I'd also like to suggest that you don't wait until he is pre pubescent or pubescent when you have this discussion. Children in puberty are already grappling with identity, finding their place in this world, and sometimes struggle with a sense of belonging. It is an age that comes with new self awareness, and where children are questioning many assumptions and are exploring a sense of self. In my mind, puberty is a special, tender, and fragile transition, and is not the best time to present information that, under our cultural lens, can undermine our sense of identity.
Above all, I just wanted to put a shout of for your husband. He IS your son's real dad. Dad isn't the person who fertilized an egg. Dad is the person who taught kiddo to ride a bike, who chased away the 'monsters' in the middle of the night, who made french toast on Sundays, who read books, gave baths, wiped bums, tickled, laughed, and loved all these past years. A biological father is the person who fertilized an egg, 8 long years ago.
Your son may express curiosity and a desire to locate his bio dad. He may have fantasies about what his bio dad is really like (remember, under our cultural lens a dad - even when having not been present for almost a decade - is still DAD. A child imposes his/her idea of what fatherhood looks like onto this silhouette of a person. In his/her mind, all dads may have parented equally if only given a chance. We, as children and adults, get pretty wrapped up in hypotheticals and stories we tell ourselves. What actually happens gets wrapped up in every story we've heard, watched, wanted, rather than What Actually Is.) It's very normal for a child to want to know more about where they came from. To know what he looked like, sounded like, what he acted like, and why he left.
Big hugs, and good luck.
Ephie (Kinship provider, everyday Mother, and child whose (favorite and only sibling) and a different bio father)