hey mate what you could do is buy a fan, you probably already have one put it on that baby, set it to low and let it blow. eventually it will break the inside walls of the stem dont know the exact scientific term for it but i did see it somewhere.
after a while the plant naturally will build a strong stem and that should solve that.
is this using sunlight or do you have your own indoor lighting system for this? honestly from the size of the stem i think it should just grow to be just fine but i want to make sure so i ask about your light.
if your using a HPS or MH light, its always very good to keep it as close as you can without burning your hand, just leave your hand under the light to test, depending on the wattage, lets say 150w HPS you can keep it probably about 6-8 inches away with a well ventilated area probably closer lets say 4 inches. 250-400+w make sure that its still close enough but about 18 inches away, if you have a very good ventilated area then you can push it a little closer.
if its sunlight none of that applies but you should still put some wind on it everyday, to strengthen the stem. also mate do not bury the stem too much or at all really, it intakes oxygen and you can easily suffocate and kill your child.
if it really becomes that bad, you can easily just get a plastic cup, what i would do way back in the day before i knew about the whole wind and stem strengthening, worked fine. get the plastic cup and some scissors or better yet a razor, cut a line
all the way to the top of the cup dont cut the cup in half, just a line on one side, however you can make a hole on the top might seem a little hard but it doesnt have to be a perfect hole, from the line use your razor or scissor to connect the line to the hole at the bottom, so now you should have a cup that should look like it was split halfway through to the hole, a clear plastic cup is perfect because it lets the light through still, now all you need to do is judge the size and cut the top parts of the cup off. once you have it sized up you can just bend the cup to open up the hole and put your plant in the hole, now it will have help supporting itself and you can easily just bend the cup again once its stronger to remove it later on.
another great thing to pump up your babies up is a Co2 generator, this can easily be made with products from your local grocery. here are some links on how to easily build them, i use about 2 in my setup, they both are 2 liter bottles and it has really improved everything as far as growth and health of my plants go, i never have issues with them falling over or just looking too weak. hope i could help somehow. take care. let us know how it goes.
i use the first site, its a bit easier to read and understand for making your own generator. peace and happy growing.
http://www.algone.com/co2_reactor.php
http://www.aquatic-eden.com/2006/10/build-your-own-yeast-co2-generator.html