Hello I am a lawyer with 20 years experience. I will try to help you with this.
If he moves in as your tenant's guest. He will acquire no rights.
Thank you for your attention. If he contributes in cash to gas/electric etc say £50/week would that give him rights.
Let me know if you will be returning to this question
Arguably it may do. Don't accept any payment from him, get it all from your tenant.