He's your boss, so you should have a relatively good working relationship with him, with lots of stuff flying and back and forth between you, so sending a 'nag' shouldn't be a problem.
I think that most people will forget or not bother doing something if they don't have a firm deadline that is communicated to them on a regular basis. When I request somebody do something I try to give a clear deadline and if they can't make it, agree a revised deadline and specify when you will start to remind them that it needs to be done.
IMO, you should not have waited passively for two weeks after the deadline to chase. You should have sent a polite reminder that he needed to review the document the week he said he would review it in and then send emails in increasing frequency in relation to how urgent it was getting. You need to make up for it now, by replying on the thread and setting a new deadline for the work and starting the process from scratch as he will need to allocate time for a task he hasn't scheduled for.
Even though he's your boss, you still have to manage the relationship as work will naturally flow both ways, so getting into the habit of being more proactive with gentle reminders before deadlines and polite chasers if it's missed will help establish what you consider to be acceptable behaviour and he will adjust to it, or tell you what he expects instead.