Dear Jacobus:

That is quite a boy you have there. We will pray that his faith and devotion will be with him all his life.

First, your son needs to understand that it does not matter whether the sin is mortal or venial -- sin is still sin. To sin venially is to slap God in the face, it offends God. We are to avoid venial sin as well as mortal sin.

In terms of the current situation, for your son to disobey you in this matter is a violation of the Fourth Commandment which is grave matter. The sin of Pride is also grave matter in this instance because the Pride is leading him to violate the 4th Commandment.

In terms of determining what sins are venial or mortal, the rule of thumb is any direct violation of the Ten Commandments will be grave matter. Also some sins that derive from the Ten Commandments will be grave too. For example, the 6th Commandment prohibits adulery, but the Church has officially interpreted this toi include fornication, concubinage, masturbation,and other sins against the virtue of the sexual faculty of marriage.

In the Catechism any sins that are designated "grave" or "serious" are grave matter.

A general sampling of grave matter is found in the Catechism:

1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: "Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother." (Mk 10:19) The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.

Your son must obey you in this situation or he will be offending God and you.

Here is the Church teaching:

2215 Respect for parents (filial piety) derives from gratitude toward those who, by the gift of life, their love and their work, have brought their children into the world and enabled them to grow in stature, wisdom, and grace. "With all your heart honor your father, and do not forget the birth pangs of your mother. Remember that through your parents you were born; what can you give back to them that equals their gift to you?" (Sir 7:27-28) 

2216 Filial respect is shown by true docility and obedience. "My son, keep your father's commandment, and forsake not your mother's teaching.... When you walk, they will lead you; when you lie down, they will watch over you; and when you awake, they will talk with you." (Prov 6:20-22 "A wise son hears his father's instruction, but a scoffer does not listen to rebuke." (Prov 13:1) 

2217 As long as a child lives at home with his parents, the child should obey his parents in all that they ask of him when it is for his good or that of the family. "Children, obey your parents in everything, for this pleases the Lord." (Col 3:20; cf. Eph 6:1) Children should also obey the reasonable directions of their teachers and all to whom their parents have entrusted them. But if a child is convinced in conscience that it would be morally wrong to obey a particular order, he must not do so.

God Bless,
Bro. Ignatius Mary