First I want to tell you that there is no way someone on this online community can determine if your guy is cheating or why he is backing off on the lovemaking.
You need to ask him why things have cooled down. Communication is the most important component of a relationship. Your illness may be a factor but it may be out of concern for you on his part. Health concerns for love ones are stressful and stress is damaging to sex drive of both males and females.
Ask him in a non-contentious way if there is something you can do to re-ignite the passion.
Maybe he is having concerns about his own health.
Talk it out. You may find he is relieved to talk about it. Starting these conversations is often difficult but so important to do.