He could ask his employer to increase the rate he is paid per mile based on the fact that he is expected to do so much on a regular basis. I agree it doesn't seem fair that he should have to pay to do his job.
Your employer does not have to stick to the advisory rate but there may be tax implications for him if he is paid more than the recommended rate. see this link for more guidance on the advisory rates.
If he cant get a higher rate your dh may be able to claim tax relief for any cost he incurrs which is not reimbursed.
www.hmrc.gov.uk/cars/fuel_company_cars.htm
Hope this helps