Contact adopted step brother?

I think as long as you are honest enough with yourself to accept that you may get no response at all, then you should contact him.
If you could cope with silence, should that be the case.

Thank you janeainsworth! Yes, agency would find him and send a letter. Just worried how he would receive the info that his mother does not want contact - don’t want to upset him. Although both myself and my brother and our families would like be in contact we have no further info on his background to give him. Like you say, best to send this in a letter and let him make up his own mind.

Oh what a difficult one and I really feel for you.
I too would want to reach out to a half-brother, but you have to face and accept the fact that he may not want to meet.
Would the agency act as a go-between?
If you contact him through them by letter in the first instance, that would give you an idea of how he feels. You would have to let him know the position with your mother.
Good luck.