Unlike some of the other people here, I have heard of your surgeon doing the DS. HIs patients seem to be very pleased with him. That's all I know about him.
As to why he feels it's a better option for you, there is only one study in the medical literature that studied matched groups of people with bmi over 50 having either DS or RNY during the same time period with the same surgeons. The complication rate was the same for either procedure, but the DS patients lost more weight. As time went on, the difference only became greater.
Since that study was published, the authors have presented date (not published as far as I know) regarding resolution of comorbidities in what certainly sounds like the same groups of patients. And guess what - again, the DS patients did much better in resolution of major comorbidities such as type 2 diabetes, high blood pressure, and sleep apnea. This comes as no surprise, as the superior results of the DS for resolution of almost all comorbidities has been seen every time this question has been looked at.
So while I understand that you had a mindset of RNY and you have seen your family members do well with RNY, your surgeon is actually looking at this in terms of evidence based medicine. And do keep in mind also that the failure rate for RNY is 30% long term, and higher for people with bmi over 50. And your relatives (whom I wish nothing but success) are not longterm post-ops yet. We don't know what the future holds for them.
If, after reading more about the ds, you still want RNY, that is what you need to tell your surgeon. He can't do ANY procedure without your consent. But I really think you have an incredible opportunity here to do better, and hope you can step back and look at both procedures with an open mind.