Go to a department store.
Macy's, Penney's, etc.
It will cost lower there.
But, $300 is on the low end.
Now, what is the typical "interview" attire in your area for the construction industry? At his level?
For example, here in Hawaii, in the construction industry per Supervisor level, they do not interview in "suits." We have our own locale specific, dress/interview style. Which is not the same for every industry.
Also important is that he get his resume and job references and letters of recommendation, in order, and updated. I assume he did that already. And sent out resumes already?
Then, do you even know what new state you are moving to?
And I assume, he will be interviewing there for jobs?
And are you going to be moving, despite not having sold your house yet?
IF he gets a job in that new state in the meantime?
Suits, a good nice one, is more than $300.
But again, check department stores, and sales. To control budget.
And don't forget dress shoes to go with his suit and tie and shirt and socks. That is in addition to the cost of the suit.