1) Lions. There are lions in this part of the world, ready to leap out of the bush at any moment and attack the unsuspecting passer-by. As he makes his way on the long, hard journey to Ethiopia, Salva is genuinely worried that this might happen to him. Thankfully, he has his Uncle Jewiir alongside him. He's armed with a rifle and has vowed to protect Salva from all potential predators, be they animal or human.
2) Hunger. On the whole, you'd have to say that this is the greatest threat to Salva. He's embarked upon a journey through a dry, desolate land where little or nothing ever grows. His destination isn't much better, either. Although the refugee camp in Ethiopia may be a place of relative safety, it's unlikely that there'll be enough food to go around there. Ethiopia itself is in the grip of a devastating famine, so there's a very real danger that Salva, like so many others, may suffer death by starvation.