DS went through this stage a little earlier than your DS & we tried the stern 'no biting, that hurts' and also putting him down/walking away but found it just made it worse & he'd try to go in for the 2nd bite like your DS. We then tried saying to him 'kisses or raspberries' when he would come at us with that wide open scary mouth! He quickly would then start to either give us a kiss or it would become a game of raspberries which he finds hilarious. Not sure if it will work for you but it worked a treat here. GL

Now, if only I could find a way to stop the hitting that has started....arrggghhhhh!