It would depend on what their doubts were about. If they were becoming discouraged because they realized the JWs were not as loving toward them and/or their son as disciples of Jesus are supposed to be, then maybe I would talk to them about finding some special day-programs (non-JW related) where properly trained and qualified people can offer them a supportive relationship toward their son.
I would do this in a subtle way, and offer to go along with them, or perhaps even give them a break now and then to take the boy. They must be having caregiver burnout BIG time, so I would encourage them to have some "couple time" that's not JW related, and also to meet other parents who have children with challenges similar to their son's. It's very eye-opening and refreshing to spend time with people who have walked a mile in your own shoes, compared to JWs who have no clue about what you're going through and really don't give a damn or don't have the time or inclination to develop a friendship with you because they "don't want you to drag them down spiritually".
I'd be careful with the "JWs are their only hope" mentality, and not even mention it, until it was an issue raised by them. It won't take long for the parents to realize that "worldly" people can offer them a sense of camraderie, unconditional acceptance and a way of facing reality without being too "in your face" about the JW religion.
The anonymous mailer - even if it includes CTRussell's quotes about "imbeciles and infants" not being worthy of God's blessings - would have to wait until they were seriously considering leaving the JWs for their lack of love and understanding and support for the family's situation. No matter what they do in their situation, they can't win. The JWs are probably horrified by the boy's presence at meetings and can't stomach the thought of having him out in service (bad advertising), and so the parents' service time is suffering which makes them "poor quality" JWs anyway, and not worthy of association outside of the meetings.
As much as I hate the idea of shattering the couple's "only hope for their son", the reality is that JWs are giving them a false hope. I think most of us felt hugely disappointed when we realized the reality, that the hope held out by JWs was merely the big carrot on the stick being held out to us so that we'd work hard at selling WT literature and comply with the rules that were being imposed on us, even though we may have thought of the JWs as the "only hope" we had too. Once we get comfortable with reality, we move on to better things.
Love, Scully