Thanks everybody, you've all given me a lot to chew on. Luckily he said in his own email that every now and then an email would be okay, but without talking of anything religious and he stated 'mutually of course' after that, so that's not a problem. He will definitely not try to proselytize, because he knows me well enough to know that I will give him the same in return to tell him what's wrong with the WTS and he doesn't want that.
Incognito worded exactly what my issue is: "If you feel as though you're walking on egg shells and can't be yourself, what is the point of getting back together as it can't be a true brotherly relationship." At this moment I could ask him a thousand questions, but I'm not sure if I will dare to ask him even ten of 'em. It's really walking on egg shells and I don't know if I can live with that. But I think I'm willing to give it a try. It's been years since we've seen each other and he knows now that I live in Spain (for him less risk that I drive over to visit him in an impulse), so even if he wasn't a JW anymore, it would still take time to get closer to one another again. It's just that I'm not sure by what and when (or if) he'll draw the line and cut me off again. And there's really only one way to find out, and that's try it. Just take the damn risk and see how far we get.
I'll just have to be careful, since he takes after my father, which is a stubborn *sshole who is easily offended (and thinks he's always right, even if you prove he's wrong), and I'm not sure if this is a genetic/family thing or a WTS-inheritance... On that account I think Jgnat's advice is helpful. I'll have to address his authentic self and keep looking for that part of him. And get him to think back to the wonderful times we used to have as brother and sister and how we always stood up for each other (growing up in a dysfunctional family we needed each other very much in the past, so there's a LOT that we've been through together). I used to be able to make him laugh a lot, so who knows, and maybe I can get him to record his voice and keyboard or guitar and add my voice and guitar to it, so we can have fun again in that way. I think that's the only way to approach this.
But it'll take some effort of me to let go of the hurt he's caused me by cutting me off. And maybe I'll have to tell him that I'm cautious, because I don't want him to hurt me again by cutting me off again. If we start this, he'll have to know I'm not a toy to play with. But I guess he'll understand that, since we have a mother who does that all the time.... attract, repel, attract, repel... That's the only good thing of me being DF'd, that I'm free of ever having to meet that woman again. But that's another subject
Anyway, thanks a lot for your input, all of you!