My memory of contract law is that there is an "offer and acceptance"
Him offering fuel for sale is an "invitation to treat"
1. He gives invitation to treat
2. You make the offer to buy
3. He accepts the offer (or not)
So in my opinion he doesn't have to sell you anything, sorry
unless there is some fancy condition in the licence that says he has too, god knows how you get a copy of the licence small print