All mothers of sons will understand the importance of teaching your child to aim properly. This is an important skill both from a social acceptance perspective as well as from a hygiene point of view!
It is important to reinforce this skill from the start. Try to encourage good habits so if your son is in watching daddy empty his bladder, remind dad not to make this a play opportunity.
Encourage your son to take responsibility from the start so if there are any near misses or little dribbles get him to wipe them up with toilet paper.