What I want to do:

I have a form with a lot of fields(nick, email, name, surname, etc.) but the user has to fill Nick and Email first in order to be able to fill the other fields(this is because we want to check that the nick and mail aren't in use by another client before he can introduce the rest of his information(name, surname, etc.)).
So, the user introduces Nick and Email and then he must press a button named "Validate", if the values are available(successful validation) then the rest of the fields are enabled and the user can continue filling the form, otherwise the fields stay disabled and an error is showed to the user.

The form will be located in a JSP, it will be submitted to a Servlet, once in the servlet I must validate the information that is in the form(i have a .JAR file included in this servlet, the validation consists in calling a function from that library, the function returns a boolean) and then I must return back to the same JSP the boolean that will represent the result of the validate function.
Now in the JSP I must enable(or not, depending on the value of the boolean) the rest of the TextFields.

I'm not sure if this is right but i was trying to submit with the button and at the same time run a javascript(onclick) that will use this boolean value that the servlet sends back to the JSP after making the validation. The javascript consists on an IF sentence that evaluates the boolean and if it's true then it enables all the fields on the JSP.

Problems so far:

I was able to send the Nick and Email from the JSP to the Servlet and to make the validation of the values, now i have the boolean but i have no idea on how to send it from the Servlet to the same JSP and use it in the onclick event of the same button I used to submit the info. I don't even know if it's possible to do this...

I'd be grateful if someone could give me a hand with this, i'm newbie in Java programming so i would appreciate simple explanations if possible.
Also, if there is a better way of doing what i want please share it, and if there are any doubts ask and i will try to explain it better.