The alleged incident happened in Victoria Road at around 7pm.
According to police, the boy refused to get into the car and the incident was reported to the police - who were able to respond immediately, as some officers were already in the area.
A Lincolnshire Police spokeswoman said: “Unfortunately, there is no description of make of car, so the extent of police enquiries is limited.
“Neighbourhood officers were in the area all evening anyway because of Halloween and kept an eye out.”
• If you have any information, call police on 101 and quote incident number 402 of October 31, or call Crimestoppers anonymously on 0800 555111.