Re: Absent Patellae
Bryan, perhaps his parents would consent to a conversation between you and his doctor, so you could gain perspective on the limitations of his condition, and the doctor could get a feel for the training regimen, and pooh-pooh any activity (suwari-waza, shikko, seiza, for instance) that he feels might be detrimental.
Also, I would definitely get his folks to sign a waiver (I'm sure you've already thought of that). One added benefit that the boy might gain from his training is the increased ability to recognize and refrain from those things that he should not do with his condition (although you don't say how old he is, so I don't know if that level of judgement is within his grasp yet).
Teaching children with special needs can be a very rewarding thing. You will often find them imbued with incredible drive and thirst for knowledge. This attitude will be infectious with others in your group. Good luck to you both.