Criminal re entry to the Uk

Hello everyone I'm really hoping someone can help me as I can't find the relevant info anywhere.

My partner came to live in the UK at aged 6 with his mum and brother from Trinidad and Tobago. He got a British passport and citizenship.
When he got older he fell in with the wrong crowd and ended up getting sent to jail for 5,5 years for drugs offences. He was not deported or forceably removed but he decided to move back to Trinidad to study for 3 years. His course has now finished and he wants to return home. His British passport has expired by 1 year. And he has an in date TT passport.
He has a child and family in the UK aswell as me and he is worried that when he tries to come back they will refuse him due to his past conviction.
The information on the home office website seems to refer to people who were deported or only have leave to remain rather than a British passport.
He lived in the UK for 17 years before he left and had family here.
My question is will there be any problems at the border? The British embassy have told him he can travel back to the UK on his Trinidad passport and present his expired British passport to enter permanently then he will need to renew it. BUT he hadn't told then about his criminal record when he asked this.
Would it be a good idea for him to renew his passport in Trinidad begore he returns? Would that flag up any issues? Or is it a case of get on the plane and see what happens?
Thank you in advance for any help

He could apply for a passport from abroad but might take a few months.
On the face of it he can't be refused entry on a British passport, unless he has been stripped of his citizenship.
https://www.gov.uk/apply-renew-passport