I'd tell him the truth. If it was just a few pounds, that would be one thing, but if he is "extremely overweight," and it turns you off, I think that eventually it will be too much of a problem to let slide. You could tell him you like him and are attracted to him, but the weight is a turnoff. Tell him that you like him regardless, but if he really wants to turn you on, he needs to slim down. Stress that you are not insisting that he change, and that you don't expect him to get ripped, you are just telling him that you will be even more turned on if he gets to a healthy, height-weight proportionate figure. Offer to help and encourage him, in whatever way he chooses (he might prefer more dieting and aerobics at home than the gym). He should be grateful for the offer of help, and the fact that you are attracted to him anyway.
If it's hard to talk about, you might write it down before you say it. You could give it to him to read, and ask him to read it, think about it, and discuss it with you shortly afterwards.
If he is not receptive, you have to ask yourself if you can stay with him-- friendship might be better in that case than dating.