It is not as much of lose lose though for him. He has far better chances with allowing a referendum than denying one, because there is every chance the result would still be a no, and there would also be no 'denying the will of the people' claims to cling to. It is a gamble. He stands to lose the Union, and support thatbhe has gained by being tough on the issue.
Saying no though, if the route above is where it goes is far riskier, because a single issue GE up is probably likelyn to result in a pro independence majority, through that mechanism rather than straight forward yes vs no. How that would then be dealt with legally gets messy.