As Exmetpolice indicates, if you now tell the police (and/or the court) that your boyfriend had your permission to take the car, you risk prosecution for wasting police time.
Further, if the Crown Prosecution Service was of the opinion that they could prove that your original statement was correct, but that you and your boyfriend were now colluding to avoid him being prosecuted, you could both be charged with attempting to pervert the course of justice. (Conviction for that offence would almost certainly result in prison sentences for both of you).
The question of insurance is also relevant. If your boyfriend was insured to drive your car (either as a named driver on your policy or through a third party provision of any policy he might hold), you can't get into any trouble (in respect of insurance) if you now state that he had your permission to take the car. However, if he was uninsured (and you state that you gave him your permission to take the car), you'll be admitting the criminal offence of permitting him to drive without insurance. Prosecution for that offence would result in you being fined (up to �5000) and receiving 6 to 8 penalty points on your licence.
Chris