To form the present continuous tense, use the helping verb 'is' or 'are' followed by the base form of the main verb + 'ing'.
In the given sentence, the subject 'he' is teaching his son to ride a bike. To form the present continuous tense, we use the helping verb 'is' or 'are' followed by the base form of the main verb + 'ing'.
He is teaching his son to ride a bike.
A: Why are you teaching your son to ride a bike? B: Because she is too busy.
Cindy usually does the shopping, but I am doing it today because she is ill.
She is swimming in the river. Ann is tidying his room.
A: Is Tom cleaning his shoes now? B: No, he isn't. He is having a party tonight.
In summary, the present continuous tense is formed by using the helping verb 'is' or 'are' followed by the base form of the main verb + 'ing'.