Mostly, I think you'll find that he will learn this in his own time. If he's not doing it now, it's either because he is not quite yet strong enough to (standing up from sitting takes slightly different muscles than walking does), or he doesn't want to.
If it's a matter of want, you can help that along by leaving him be when he falls over or is sitting. He will eventually decide to do it on his own, maybe just not as fast as you'd like.
If it's a matter of strength, to some extent this will just come as he gets stronger and as he walks more (which works some of the muscles needed, though not exactly the same set). You can help by making sure he has a lot of opportunities to work on his abdominal muscles (the "core" muscles, in the area of the belly button, mostly). Try not to use "bouncers" or other similar assisted-standing devices as much as you can; keeping that time to less than an hour a day is good (that's what my pediatrician recommended) as those keep him from having to develop his own muscles. Let him walk as much as you can, and let him try to stand up by himself at least for a while before helping him - the more you let him do, the better off he'll be.
If this goes another few months and he still cannot stand on his own, talk to your pediatrician about it, both to get more specific-to-you advice and to make sure that there aren't any developmental concerns. I don't mean to suggest that there are at this point, but your pediatrician will know best, and will have good advice for how to help your son specifically.