Thanks Devonlady. He's gone to what I hope is a good and knowledgeable home.
I ended up giving him away, which is what I expected. But I didn't want to advertise him initially as "free to good home" and thereby attract a load of wannabe horse keepers who had no idea of the potential costs or commitment involved in keeping an animal.
The gentleman who came to see Toby initially said he thought he'd be too small for what he wanted. But you could tell he really liked him, and he was impressed at how calm and laid back he was for such a young horse. So, after a bit of thought he decided he would have him after all. You can't guarantee a good home, but I feel happy that he's gone to a knowledgeable animal lover, and hopefully all will turn out ok.