It is unlikely to be too long, my guess would be days rather than weeks, the youngest person I recall was 16, but they were not also suffering from acute leukaemia. I suspect there is no attempt to actively treat either the bleed or the underlying condition. There's a good chance the bleed will extend, and affect the cardiovascular centre in the brain or he will develop an overwhelming infection from a line or in his lungs and potentially develop multi organ failure. Even young hearts can't keep beating it other systems fail.