The law around paying for sex with trafficked persons whether you know they are trafficked or not came in in 2010. But payment would have to made.
The law around having sex with a trafficked person, whether you knew they were trafficked or not and whether payment was received or not came in in 2015.
So he'd have to have eligible offences after those dates.
Prior to that the sexual offences act 2003 recognised all persons engaged in paid sex worker aged 16 and 17 were victims of abuse rather than being prostitutes as they would have been deemed as before but the laws were around penalties for people causing them to be engaged in acts of sex work, the pimps and traffickers, not individuals paying for sex with 16 or 17 year olds who could easily say they thought they were over 18 anyway.