I am the father of a 3 year old son. should i potty train him to pee standing up from the beginning or teach him that later on? what is the best way to train him to pee standing up?

I'd do whatever you think he's going to enjoy and respond to. You can train him to do both ways, or one particular way if he feels that's better. 3 years old is the average age boys start to train, and generally he will follow whichever model he finds easier. Sitting makes less mess, but standing is going to be easier in the long run, and you have to get there anyways.

As the father and male role model, you have an undeniable advantage in demonstrating “standing up”. Children love mimicking adults. Be patient and avoid punishing accidents. Best wishes.