You haven't said whether the form has been lost in the post or his current award is coming to an end and you were expecting a form before now.
Forms are typically being sent out a year in advance but plenty of people are finding theirs arrive with less than six months ago or even with just a month to go. It is impossible to tell exactly when people will receive them.
If you son's award is coming to an end in the next three months - I would ring the DWP and ask when you will receive it. That will put your mind at rest.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems