You should not begin potty training until your child has developed some control. If his bowel movements are very predictable, you might want to start there. If not, or if he goes only once a day, you might want to focus on urination. Most children learn either way, eventually, without significant difficulty.
Troubles can develop in some children no matter how they are trained. Children who learn to hold onto their stool can develop stool retention and constipation. Other children cannot stay dry at night and continue to wet the bed for years. You can talk to your child’s doctor about these problems.
If any approach significantly upsets your child, you may just need to take a break from training and return to it later.