If you agree to do it I guess his prospective new employer will write to you with a specific set of questions, one of which might be 'would you employ him again?'. I doubt he has told them you let him go, which I gather you did. Kids do a lot of maturing in their 20's and he may have learned something positive from his experience of working with you and the mistakes he may have made. I don't think you should lie but neither put him down so he has no chance of getting the job.