HTML after label radio button (depending on radio button)
Hope this will help. please follow the comments.

 function CUSTOMMODULENAME_form_alter(&amp;$form, &amp;$form_state, $form_id){
        if($form_id=='YOUR FORM ID'){
           /* uncomment the following code after updating the key variables&lt;field machine name&gt; and check out the option keys.
           */
           // echo '&lt;pre&gt;';
           // print_r($form['&lt;field machine name&gt;']['und']['#options']);
           //exit;
            $form['&lt;field machine name&gt;']['und']['#options']['&lt;key&gt;'] .= '&lt;h3&gt;Hello World &lt;/h3&gt;'; 
            // the above code will add &lt;h3&gt;Hello World &lt;/h3&gt; alone with the existing radio button label
//Remove the comments if it helps.
        }

    }


Let me know if you need any additional clarity.

Thanks