The age really does not matter so much. But if you bond him to a very old rabbit, say 6 or 7 years old the older rabbit will eventually die and leave him alone. Bunnies can die from sadness…it is true.
A younger one who is an adult maybe under 5 would be okay. A baby would not bother him but two unaltered males might fight as they get older. A baby female would work if you want hundreds of babies in a few years which you probably do not.
The female is usually more dominant and territorial. So it would be best to not allow her to establish her own territory before the introduction.

I once had a 10 year old rabbit, and found a young rabbit (less than a year old) that someone had thrown out. Our 10 year old male rabbit was already neutered, and the female rabbit we found was spayed shortly after we took her in. They bonded quickly and only had 2 years together before my senior rabbit passed away, but they really loved each other.

I see that you are in London. In the U.S. almost all rabbits in rescues are already spayed and neutered before they are adopted out, which can save the adopter a lot of money. I did a search and see that there is a rescue in London, Greenwich Rabbit Rescue. I didn’t see if they spay or neuter or not. Since you said you don’t have the option of adopting, I thought that you might not know about this place. http://www.greenwichrabbitrescue.com/rabbitsthatneedhomes.htm