No. While I would feel bad for the kid and be trying to get him help, the reality of the situation is that I would get arrested by the police long before I could provide any miracle help to cancer. That shit takes months of treatment at best to have any meaningful effect if it is going to have one at all. The police would catch up to me in less than a day, since people tend to get rather worked up over kidnapping and the hospital staff have no reason to protect, since they are then considered accomplices. So yeah, logic of the situation says calling child services is the better plan.
Also, some people may want to forgo any treatment at all. A lot of doctors who are diagnosed with cancer actually undergo no treatment because they know that generally the best case scenario is a couple extra months of living, but living in pain, nausea, fatigue and being generally uncomfortable. Just putting that out there.