The defender, 17, moved to the Cherries last year from Charlton Athletic.
William qualifies for England and Nigeria internationally and told afcb.co.uk: “I am delighted to sign my first professional contract. It’s a good moment for me, my family and everyone that has helped me get through.
“Playing for the development squad has been very intense. I was involved in the pre-season tour to Spain and we played some good teams.
“I am also getting used to the style of play. It’s full-on, press, press, press and intense. It demands a lot.
“The quality of the opposition is high as well. Other teams can have first-team players and older players, while in the youth team you only play against people of the same age.
“As the games are full-on, there is a huge emphasis on how you recover. You can train and play two very good teams in a week so you need to know how to recover.”