Boy built physical relation on promise to marry now denying it

Physical relationship on the pretext of marriage is not a simple cheating but playing with the modesty of a woman. Hence the courts have taken a strict view in this and in your case as you say that even both of your family members met for marriage talks. Under these circumstances it is an offence of committing rape/sexually abusing a woman. For such offence he could be prosecuted under section 376 of IPC and could be put behind bars not less than seven years. So you better file a police complaint and the prosecution will begin the investigation.

- Section 376 of IPC

If a boy established physical relations with you on the pretext of marriage and then he refused to marry,you are required to file criminal complaint in police station ,sho, dc p commissioner of police. Then meet all such officers for registration of fir. If they fail to register fir, file criminal case in court .

You are free to lodge an criminal FIR for physic al relation on false pretext of marriage.. go to concerned police station and lodge an FIR against him as per law.
Police will take action and lodge an FIR under section 376 IPC

This is clear case of rape on pretext of marriage, you can file fir against the said boy but as the trial of the same may take 2-3 years and during the trial there may arise many awkward situations so if you are mentally strong enough to face all these than you must file the case with your police station