Currently AWS ELB doesn't support Websocket balancing, there is a trick to do it via SSL, but it has some limitation and depends on your app logic. So if websocket connection is used only as server-client communication, it will work. But if you have more advanced logic when clients must communicate with each other via a server then this solution won't work. For example one client has established connection for a chatroom, then other clients can connect to the established chatroom and communicate with each other.
Then only possible way to use HA-proxy http://blog.haproxy.com/2012/11/07/websockets-load-balancing-with-haproxy/
But shown example just shows how to configure HA-proxy base on two servers. So if you do not use Amazon Autoscalling Group, the solution is good. But if you will need use ASG, the question about add/remove instances to ha-proxy config is other challenge.