I am trying to use the new reCAPTCHA capability on a Web-to-Lead contact form that is being displayed on a Weebly website. I would like to enable my Submit button based on successful result form the reCAPTCHA. Right now, the user can still click the Submit even if they haven't check the reCAPTCHA checkbox. A lead isn't actually created, which is good, but it just goes on to my "Thank you" page, giving the user the impression they have submitted their contact  info. 

Here is the HTML at the end of my Web-to-Lead form that handles the reCAPTCHA. callValidation() is a function that checks to see whether the user has entered required fields in the form.
 

<div class="g-recaptcha" data-sitekey="xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"></div><br>
<div id="button">
  <input id="sell_house_submit" type="submit" name="submit" value="Get My Free Quote!" onclick="return callValidation();">
 </div>

I'm not an HTML/Javascript wizard. Can anyone help me?

TIA!