You can activate one of Sorin, Ravenous Neonate's loyalty abilities the turn he enters the battlefield. However, you may do so only during one of your main phases when the stack is empty. For example, if you control another permanent with an ability that triggers at the beginning of your postcombat main phase and that ability is on the stack below Sorin of House Markov's last ability, there will be an opportunity for your opponent to remove Sorin, Ravenous Neonate before you can activate one of his abilities.