Angie, this is a perfect time to have your son start, since he will need to use the toilet independently in kindergarten. It would be great for him to begin kindergarten feeling confident doing it.
Dry toilet paper can be scratchy, irritating and difficult for young children to manage. Instead, you may want to have your son start by using damp, flushable wipes. Use hypoallergenic wipes, which can be less irritating. First, have your son practice wiping a part of his body that's easy for him to see and reach. For example, you can put some baby powder or chocolate pudding on his knee or arm and have him practice wiping it off. Then teach him how to reach behind and wipe his bottom, throw the wipe in the toilet and flush.
If your son's bottom remains irritated, a little petrolatum jelly dabbed on his bottom can help. You can keep it by the toilet and teach your son to scoop up a dab with clean toilet paper and smear it on his bottom after he wipes himself. You can also put a dab on his bottom after his bath or shower. Be sure that your son doesn't use bubble bath or soaps with perfume or antiperspirants since these can be irritating to the delicate tissue of a child's bottom.
Also, make sure your son knows to wash his hands with soap and water after using the toilet. Keep a stool or a couple of phone books on the floor in front of the sink so he can reach it easily.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.