Firstly, he's failed to complete the job, meaning he has broken your contract. That's a Civil matter though. I guess you don't want to get involved with the courts etc.
The heavier point is that he's required by law to provide a safe installation. Theoretically, how do you know it's safe? You have no evidence. That puts him in a difficult legal position.
Being practical ... Google "Gas Safe" and contact them right away. They should take this very seriously, at the very least threatening his registration.