Custom field "required" validation
Probably a stupid question but have you tried using the #required setting when you created the field i.e.

'#type' =&gt;
'textfield', 
'#title' =&gt; t('Subject'), 
'#default_value' =&gt; $node-&gt;title, 
'#size' =&gt; 60, 
'#maxlength' =&gt; 128, 
'#required' =&gt; TRUE,


from Form API reference https://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/7#required