I was shocked when my 16 year old's friend told me takes steroids for playing basketball tournaments. What should I do now

It is a matter of concern. Steroids have very serious long term consequences and you do not want you son to go through that. You need to handle this in a very subtle manner so that he feels the need to confess this to you. otherwise you may have to ask him directly because too much delay might lead to further issues. Speak to your husband also.Together you may be able to find a solution and solve this problem.