Firstly, is there a reason why he has not put the Van into the Business, my understanding is that if it is used 100% for business in the Ltd Company, that the purchise price of the van will receive 100% Capital Allowance.
Therefore, all receipts for fuel for the Van are allowable against tax. In terms of VAT, the amount of VAT on the purchase of fuel should be detailed on the reciepts. I have read before that LPG Fuel is 5% VAT, but this will be listed on the purchase receipt too.