Thanks for the advice. Having done quite a bit of reading on my own, I think that in some ways he is correct, the risk posed to him by taking MDMA is actually not very different, if at all different, to the risks I take by drinking alcohol. (There doesn't seem to a single study that proves its harmful in the long term where the participants have taken only MDMA rather than MDMA along with other drugs - it has been implicated in deaths from things like heat exhaustion of people who drink alcohol and dance lots and don't drink water - he doesn't mix it with alcohol). So I don't really feel justified using that as an excuse as to why I think he shouldn't do it. (The fact its illegal, whether he agrees with it or not, is more of an issue, in my opinion, and the fact he isn't really himself when he's on it).
And yeah, I'm willing to give up the festival if necessary - I've been to festivals in the past and enjoyed it, but this particular one is known for being really fun but a bit 'druggy'. If I didn't go with him, the only other person I know who might go is a close friend who will probably also take drugs when he's there. Also its sort of his 'thing' - he's been the past two years with firstly his ex & her friends, and last year with his sister, so if I choose not to go they can go as a pair again.
Now just need to bring it up... Thanks!