I agree, if the story is true he should just do it. Every time you delay you give opposition time and opportunity to mobilize and marshal forces. For instance recently Texas tried to pass a bill to keep the creeps out of girls bathrooms, locker rooms and showers. They dragged it out & the left (including the NFL) used every tool to stop it.
Passing a bill in the legislature requires time and debate. However, the “Iran Deal” has no real legal status. Trump could say, “No deal. Bye!” right now.
What that would mean in terms of policy, of sanctions, or whatever, might take a few days to work out. However, it would be a clear statement that we are not going to support Iran while they develop nuclear weapons.