A friend of mine found out a few years ago that he had a FULL brother, who had recently contacted his mother for the 1st time aged 36. That was pretty weird for him!
They get on OK, look very similar, but have quite different experiences as the product of their upbringing. Fortunately, the mother welcomed him back into the fold.
As you wisely have said, you recognise the shock, & bizarre emotions your potential sibling is having - he'd obviously liked to have discussed stuff with your mother, but that's impossible.
From what I've witnessed, everything should progress slowly. You haven't mentioned whether or not you have a strong relationship with your adoptive family, or whether you have their support - whatever happens, they're the people who shared their life with you - you're literally a stranger to this other guy - & suddenly he finds out that he shared a mother, & she wasn't wholly truthful with him, which will completely alter his perception of her.
I genuinely wish all the best for you. You'll be full of a desire to find out as much as you can about your mother, but that might be a little overwhelming for your brother, so just concentrate on talking about your own lives as a way of information manifesting itself, & building a relationship together.
I hope you get on, but sometimes these things don't work out at all. I know one guy, adopted at 3, who met his mother 20 years on, was overwhelmed, moved back in, then moved back out in a week, & has never spoken to her or his siblings since.
You've done all the right things, but try to keep your expectations low, for your own sake.
Good luck.