What's the pay structure? How much, how often? Direct Deposit? Salaried or hourly? Bonuses?
What are the benefits - health insurance, dental insurance? How much is his premium? Do you need to change doctors? Will your coverage and expenses change? For the better or worse?
Other benefits...paid time off? Sick days? Vacation days? Life insurance? Disability?
Disability insurance would be a big concern to me for someone working on the road, especially in a hands-on job. Make sure there is good coverage available if he gets injured.
Is there a union? What are the dues? Does he have to join?
Is there a retirement plan?
How long has the company been in business? Who are the clients? Who owns the company? Are the owners trustworthy people? Have they been investigated for anything? Are they solvent? How do they grow their business? How to they attract clients? How do they retain them? How long do their clients stay?
If it's a public company...look at earnings reports and other filings. Google them and see what others say. If it's a small company, rely on the contact he has to answer some of this.
Why is the position open? If someone left...why did they leave?