Police were called after the incident outside McDonald’s in Orion Way, Kettering, just after 7pm on Thursday, May 16.
The victim saw the unknown offender appearing to tamper with his vehicle and went over to challenge him.
But the attacker then punched him in the face, knocking some of his teeth out.
The victim fell and broke his ankle.
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A police spokesman said no arrests had been made and that there was no description of the suspect at the moment.
Witnesses or anyone with information should contact police on 101 or Crimestoppers anonmyously on 0800 555111.