Nunja worry, things will work out all in it's own.
If you think you're hiding anything from anybody, you are sadly mistaken. Sean definitely knows things feel off - he sees your forced, fake smile, your tensed shoulders, the fake laughter and uneasiness between you. He just doesn't know why.
He probably spends every other weekend with you and your new family. He sees how easily you interact with Connor, the relaxed and genuine style you have with your new step-son. Sean isn't stupid, he's just young.
And so, time will pass and the weekends with Sean will become more strained and uncomfortable for the two of you. You'll justify that with spending more time with Connor to make yourself feel better and to just relax.
You won't miss the skipped every other weekend visits when Sean has other commitments or obligations that fall on your visitation weekend. He'll tell you "Dad, this is important, can I come another time?" and you'll say, "No problem, see you soon."
And then every other weekend becomes every other month. Until he's about 16 and decides not to pretend to want to spend time with you and - just doesn't even bother trying.
And you'll be ok with that.
You'll get the obligatory invitation to his graduation, but miss his first prom experience, teaching him how to tie a tie and choosing a boutonniere. You'll miss his first kiss and introductions to his friends and school accomplishments. He won't bother inviting you, he knows you don't want to be there.
He'll live his life without you and be none the wiser you don't love him.
Other than, he already knows. Even at the tender age of 13.
So, go live your life. Sean will be just fine without you. Or the reminderence that he's not Connor.