Another stepdad here, and here's a different perspective.
Little Unicorns 1 and 2, both boys, were 7 and 9 when I first met them. Their father is around and although he'd been a dick to Mrs Unicorn (multiple affairs), he did actually care about his boys and was around. Over the years, though, his interest waned a bit with his 2nd, then 3rd wife, 2 more kids with 3rd wife, the boys grew a bit more distant. We always had an amicable relationship without a single bad word said about biological father or either of the new wives to the boys. In the meantime, we also had Little Unicorn 3 ourselves. LU1 and 2 and I had a great relationship to start with, had the usual teenage wobbles like any other parent, then came out the other side stronger than ever. Life is great. As far as I'm concerned, they're my sons and they treat me as if I'm their dad. However, they've never called me dad, just my first name.
Anyway, fast forward a decade and a bit, and LU1 was 22, at home while I was working from home. We have a Ring doorbell, and when it went off, as I had just managed to connect to the video and sound, LU1 had opened the door so I ended up listening in. It was a man who asked for me by my name saying to, the adult, LU1 "are you Mr Unicorn?" LU1's instant and completely natural reaction to the question was "oh no, that's my dad."
Somehow, being called dad when I'm not even there to hear it actually made it even more special for me. There was no need, there was no complusion, it was his choice, and his choice as an adult to refer to me as dad. It was one of the greatest moments ever. I did go and see him afterwards and gave him a huge man-hug. I think you should feel immensely proud that your late-teen stepson is referring to you as dad when you're not there, because, to my mind, there's no greater compliment.
Take care and be proud of what you have achieved.