If he was to win the lottery, and he did not want to share the winnings, you would likely have a difficult time making a claim for the portion of those winnings. Based on what you have written, you have given a gift with no conditions. If you want to protect yourself, put something in writing.
DISCLAIMER: Please be advised that this post is not intended to constitute legal advice and is for informational purposes only. This posting in on way creates an attorney client relationship. You should contact an attorney to protect your interests.