My oldest son started standing to pee around the age of three. We taught him how to do it since it was something he was eager to learn. We found activities such as 'Peepee Dance' to be helpful. This is a fun game where you moonwalk or spin while peeing standing up. We also set him up with what he needed to do it successfully—a little potty chair and toilet seat adapter, something his age could reach, and a steady basis of praise for his efforts.
At first he was a little clumsy and there was some inevitable puddles. But as time went on, he became more comfortable with his new potty routine. We continue to reinforce the importance of aiming and accuracy, letting him know that he can do it if he concentrates.
Our other son who is younger learned to stand up to pee at around the age of four. We followed a similar process for him and he has now become quite the expert. We still, however, remind him to stay focused and be careful before and after doing it.
In either case, I recommend doing some research to learn the best way to teach your son this skill. There are some great resources out there, such as Shelly Torgove's 'The Miracle Method' or the hints and tips on Potty Training Solutions. You can also read real life experiences shared by other parents on sites such as whattoexpect.com or netmums.com.