“Can parents teach emotional regulation in preschoolers or do I need to take him to a therapist?”

There is no need for you to take your child to a therapist to teach him to regulate his emotions. It is fantastic that you are consciously thinking about how to teach him about emotion regulation. Emotion regulation is one of the most important skills for our children to develop. In general, children who can regulate their emotions, tend to do better in life. Emotion competence, social competence, and academic achievement are very closely linked.
How can you teach him? First, remember that children learn more from what we do than from what we say. So, if you want your child to regulate his emotions well, be a model for him. Show him how you do it. Second, teach him explicitly about emotion regulation. Whenever he loses it, help him to calm down, and then talk to him about what he is feeling, why, and how to handle those emotions. The idea is not for him to avoid feeling negative emotions, but for him to be able to deal with those emotions appropriately. Third, children tend to repeat those behaviours that their parents like. So, use praise whenever he regulates his emotions effectively, and ignore him or tell him when he does not.
I hope this information helps.
Here are a few articles that you may find helpful.
Fostering Emotional Intelligence in Children: A Guide for Parents
How to Deal with Tantrums in 5 Year-Olds?
Executive Functions: Discover Everything You Need to Know
I wish you both all the very best.
Love,
Ana