Dahon handle post hinge loosens too quickly
As with any critical fastener on a bike, the vibration and flexing forces from riding will eventually work threaded parts loose. Normally this is the sort of thing that occurs over a few months--not every week. Fortunately, there are a few options that are applicable to this and any other situation where threads work themselves loose. (Before you do anything, though, make sure that the rest of your steering components are safely adjusted.)

The easiest thing would be to buy a tube of Loctite (or comparable) thread locking compound. It is cheap and easy to find. You'll want to get a formula that isn't permanent so that you can make adjustments down the road. Back the nut off until you see some threads and add one or two drops those threads. Adjust the tension where it needs to be and let it sit for a little while. (Loctite is safe for threads mostly everywhere on a bike!) In less critical applications, beeswax is a great natural compound for securing threads and preventing corrosion.

A second option would be to replace your 10mm nut with a lock nut. These are nuts with a nylon (or similar) insert in the aft part of the threads which prevents it from working loose. Similarly, you could procure a lock washer which will prevent the nut from backing out when tightened properly. (It doesn't look like you have enough space for it, but a second nut behind your primary nut (called a jam nut) will keep the adjustment stable as well. Jam nuts are typically narrower than a standard nut so they use less threaded area.) Any well-stocked hardware store will have suitable metric hardware. 

If those options fail you, you may need to take more permanent action. Either chemically (with a permanent thread locking solution), or mechanically by installing a castle nut. A castle nut has notches along one side to mesh with wire or a cotter pin that is drilled through the threaded bolt. The wire prevents the nut from backing off, but the downside is that this requires drilling a tiny hole straight through the bolt (which would require you use a drill press and lots of diligence). I really don't think you'll have to resort to this last option, but it works.

In any case, continue to be vigilant in inspecting those parts for wear and adjustment.