I'm in your situation. Almost exactly, as a matter of fact. My ex and I split for good when DS2 was 2 weeks old and he hasn't seen him since he was 6 months old (ds is 2.5 now).
My ex is a great guy, in most ways. He's got a good heart and he's made a lot of improvements over the last couple of years, but he's not able (or willing, maybe) for a lot of reasons to be a *father* to ds. Geography is a big one, money is another, plus he's a single dad trying to take care of his older son, so there's a lot of cards stacked against him. But that's his life to take care of. I don't blame him for wanting the best for his older DS. I love them both and want them both to be happy and secure. But I decided a long time ago that I was not going to bend over backwards or facilitate a relationship between DS2 and his bio-dad. I've left the door slightly ajar and it's his responsibility to pursue a relationship, if he wants to do the work to make it happen.
I met DP when DS 2 was almost 1yo. DP is the only father he's ever known and they have a wonderful bond. We've talked about DP adopting the boys after we get married (my oldest is 9 and his dad has never been in his life, he was my old h.s. bf....), but for now, we're kind of living our lives without legal "labels", which sometimes makes me nervous, but not enough to want to go to court.
The way I see it, DP and I are the boys' parents. We provide the roof over their heads and the food in their tummys, help with homework, juggle work and family when someone is sick, get up at night when someone can't sleep or has a nightmare. DP's the one who drives the boys to sports practices and has tickle wars with them and does bath-time and teeth-brushing, etc. We are the ones doing the parenting, the protecting, the guiding on a day-to-day basis.
I don't know how we're going to explain this situation to DS2 when he gets older. We've talked about it a lot, but you can't plan your future like that - there are so many unknowns out there - we've decided to follow DS2's lead and just be honest with him.
As for my ex trying to come back into the picture, we're not trying to exclude him so much as just making sure that he can't just waltz in and out as he pleases. If he wants to be part of DS2's life, he's going to have to work at it and have a plan. He asked a few months ago if he could "stop by and see H while I'm up that way on vacation"....... Um, NO, you can't. Sorry, but DS is not some old college buddy you can just drop by and hang out with and then leave and not see for another year......
When and if he ever gets ready to make an honest go of it, then we'll deal with that like adults, with DS's best interests at heart.
This post is getting long and doesn't really do any justice at all the complicated emotions that surround this subject. It's a very sad thing for me sometimes. I didn't think I'd ever be in this situation. On the other hand, the boys have a great life and they don't have anyone in their lives that is untrustworthy or flaky. They know that their family (and they have A LOT of family between me and DP) can always be counted on and that no one is ever going to bail on them or be too busy for them, etc.
So, you're not alone
There are some others on this board who have similar family dynamics, too.
Oh, and bczmama's right, short of step-parent adoption, there's really no way to legally "protect" your family unit. Your DH could walk away tomorrow and not be responsible for your DS, but that's a leap of faith you make in any marriage. One thing I did was up my life insurance lately, so DP and my parents would have $$ to hire good lawyers in case something were to happen to me. With each year, too, your case for "continuity of care" gets stronger and stronger.
What helps me sleep at night though, is the thought that by being reasonable with my ex and maintaining open communication, the chances of him suddenly showing up with a court order and a team of lawyers is HIGHLY unlikely. There are a dozen scenarios I could think of happening first (like monkeys flying out of my be-hind)