Ah I'm sure he'll be fine, 8 years is a long time to get to know each other. He might be shocked at first, just like you were, but the more he thinks about it the more he'll warm to the idea.
Ours was planned but when my hubby came home from work I told him there was something wrong with the oven and asked him to look at it. I put a bun in there and it took him a little while but he got it!
When we told my step son I got a tshirt printed saying 'the worlds best big bro', it took him a while to click aswell but it was so nice when he realised!
Good luck telling him, you never know you might get the best reaction ever!