My 14yo son broke his collarbone, the er sent him home in a sling, i got him a figure 8, can he use both at the same time & can he sleep with it on?

Comfort is key: It doesn't really matter which treatment you use as long as he is confortable. The fracture is going to heal the same whether you use a sling, figure of 8 or nothing. Comfort is the best judge.

Figure 8 is better: I've seen patients have more comfort with a figure of 8 splint. They can move and be physically active. Once that is placed properly, he won't need the sling.