There is a lot more that's structurally wrong with your situation than you think, and based on your tone and framing you yourself appear to be part of the problem. Your company appears to have an immature organizational structure without mutual respect between roles, and various people involved in this situation seem to be communicating poorly. Only ...
Besides for dns's suggestion to add it to treat his submission like any other pull request and test it thoroughly, you also need to add values to your concerns.
Create a document explaining the cost, in man hours, of each concern:
We'd have to rewrite everything
That means X coding hours, X testing hours and X debugging hours.
It's not great code
You could treat his submission like any other pull request: give him feedback like you would any other code review. If he understands the conventions of your team, if he knows your team doesn't put code into production without acceptable test coverage, then he might take the feedback seriously and improve the code.
Code quality is, however, only part of ...