I love my adopted step brother should I tell him?
Can you define what loving him is? I am guessing you have romantic feeling for him. It would help to know how old both you and he are.
If you are asking about sexual feelings you have for him, again, it is important to know how old the two of you are. There is no medical prohibition for two people not related by blood to be sexually active. Close sanguinity does not exist so any child conceived by the two of you is not at risk. You should understand that the possibility of conception is a real issue for the two of you, regardless of how careful you are to use birth control.
I don't know where you live, but there could be legal prohibitions to such a relationship. If there is such a prohibition, then you should keep your feelings to yourself.
Another consideration should be your family dynamics. Is this something you can discuss with your parents? If not, and you were to reveal your feeling for him and he was sexually interested in you, then you would have to enter into a very secret relationship. This would indicate that it is probably not a good idea to reveal your feelings and act on them. Perhaps your life would be better if you found someone else to love.
Take a chance.
You guys are biologically unrelated, so there's nothing particularly wrong that could come out of it. Circumstances have brought you a little closer than you might have wanted, but don't let that hold you back.
One of my friends' parents were actually step siblings before they started a family - with two children, in fact: my amazing, helpful, supportive friend, and her sister (I don't know her well - you know how it is between you and a friend's sibling - hint: slightly awkward).
Techically, if your parents had remarried in a different way (not to be insensitive), you could've been strangers. You could've had a different step brother.
It doesn't really make a difference.
And if an opportunity like this one comes your way, naturally, you should take it. Because if you don't, and part of you wanted to, you might possibly regret it later on...
So go ahead.
Tell him you love him.
There is nothing wrong here, since the two of you are not related, it is not incest.
So I would say what I always say about telling others you love them. If you feel that he loves you, then ask him how he feels about you. But keep in mind that if he is not in love with you he may not interpret this as good news, so with this in mind, get to know him. Spend some time together. And after he holds your hand a little, then talk about it with him.
The only unusual part here is that no matter what your relationship is, you are likely to live together in the same family. Keep that in mind.