This may sound a bit extreme, but you could discuss the company no longer paying for his fuel. Whilst this may sound expensive for him, if you sit down and do the calculations, you may actually find he is better off this way- particularly if you intend to increase his salary as a result of him losing his fuel benefit.
He could then charge the company for any business mileage he does, using the advisory fuel rates.
The other alternative is that the company continues to pay for all of his fuel, but the company charges the employee for all of his private mileage- again using the advisory fuel rates.
In either case, the fuel benefit could be reduced down to nil if you can meet all of the requirements.
Please not that due to the frequently changing fuel prices, HM Revenue & Customs regularly updates the advisory fuel rates, so always check their website for the latest figures here:
Your local TaxAssist Accountant would be happy to discuss this in more detail and calculate the differences between each option for you.