The easiest way to teach him how to water his plants is to get him a small watering can with a long spout. This will be easy for him to hold even when full, and the long spout will minimise spills. Show him a few times just how it is done, and then make it his responsibility. He will not know exactly when to water the plants though, so just remind him gently.
As far as feeding your pets goes, this will really be exciting for your preschooler. However, you will have to remind him when to feed and water the animals, how much food to put into the bowls and when the water bowls should be filled.
Safety is important here. Teach your child that the animals shouldn’t be present when he is filling the bowls, and that he should never tease the animals or try to take their food away from them. Animals can become aggressive if you try and take their food away from them.