We started with sitting first as it was easier when it came to poo as well. Then progressed onto standing once he worked it out. My DS2 quite often doesn't poo and wee at the same time, so he will often do a wee, finish, then say 30mins later, go and do a poo.
If you are outside and it's a nice day, you could get him to wee on a tree or lemon tree. That way he can practice standing up without it going all over your floor
Another trick is to put a table tennis ball in your toilet and when he goes, get him to aim for it, so it teaches him penis down.