I met Billy a little over a month ago. He spent a lot of time letting the kids on my cross country team get his autograph and take pictures with him after we heard him speak. He is a very kind man and have done enormous amounts of charity work over the years.
Asking about his trips to the Olympics was a nice suggestion. He has been to every Olympics since the one he participated in.
You could also ask him what he did with his gold medal (he gave it to his grandson), which would give him a chance to talk about the medal and also his family.
Honestly, you can probably talk to him about anything. He has plenty of life experience and has spoken with so many people that I'm sure just about any topic would lead to a wonderful conversation.
He left everyone one on my team, parents included, feeling lucky to have spent that evening with him.