Hi there. For me it was the other way around - I really wanted the bypass but my care team determined that it was medically contraindicated for me. I tried hard to change their minds but in the end I went with their recommendation. I was disappointed, but it turned out okay, I didn't have any major complications and the surgery has been effective so far.
However, in my case, there was an actual medical reason why the bypass was extra-risky for me. I got a 2nd opinion and the 2nd doctor agreed it was contraindicated. In your case, it sounds like the surgeon just feels the bypass will be more effective? If there is no medical contraindication, it seems like after being told the pros and cons, you would be allowed to choose the surgery you want. What would happen if you just said, "Look, I understand your reasoning, but I'm not comfortable with the bypass. I want the sleeve."? Would he refuse to do it? Could you get a 2nd opinion? Maybe if another doctor indicates that the sleeve is a viable option for you, your surgeon will relent. Have you shared your concerns about the bypass with your surgeon? Maybe with more information, you'll be more comfortable with it?
In any case, it doesn't seem right for you to be forced into a surgery you're not sure you want. You're the one who has to live with it, so you should be calling the shots (assuming there's no medical issue preventing the sleeve). I'd revisit the issue and get more information, then decide for myself one way or the other. Hope it all works out for you!