Make sure that your bathroom is equipped with a stool, so that he can reach the hand basin, and then help him to use the soap. Getting a supply of special towels or wipes to be used after the potty is another way to encourage your child to wash his hands after using the potty, and giving him a reward, in the form of a sticker, or even a hug, can help to reinforce the good behaviour.
Try to differentiate between using the potty and washing hands, if your child is still potty training, by having separate rewards, so that your child still achieves something if he forgets to wash his hands, but remembers to use the potty.
Older children can benefit from being taught about how germs can make you sick, but you also don’t want to overemphasise this, as it can cause your child to develop a phobia.
Remember to tell your child that he needs to wash his hands after touching anything that could have germs on it! That means after he coughs or sneezes, after playing outside, and any other less than clean activities as well.