Does his Mom think he needs therapy ,maybe he is just bragging about the cliff, best to ignore it as if he thinks he is getting attention saying he would like to jump off if you say nothing maybe he will give up talking about it ..Perhaps he sees the younger sibling getting more attention from you, and he wants some.Focus on his positive side and praise him when you see him doing something right,.very often the older child does get less attention than the younger sibling, it creates a jealousy ..good luck