Dear sister in Christ,
It brings me great joy to be of help to you, so please feel free to write to me whenever you have any doubts. Responding to you is never a burden for me—in fact, I do it very willingly because I care deeply about your spiritual growth and that of your children.
Now, to answer your question. In the well-known text titled "Per meglio confessare" ("To Confess Better") by A. Chanson (a manual for confessors from the time of Pius XII), the topic of what is permissible and what is not between engaged couples is addressed. Chanson distinguishes between two types of kisses: passionate kisses (open-mouthed) are grave sins because they obviously provide carnal pleasure, which is only lawful for married couples. On the other hand, chaste kisses (closed-mouthed, what I would call "little pecks"), according to Chanson, can be tolerated if they are given as a sign of affection, without the intention of seeking carnal pleasure.
However, I share your view: it often starts with a little peck, and then it becomes impossible to stop, as passion is strong, and one may end up committing grave sins—what are known as acts of uncompleted lust or even fornication.
If I had a fiancée, I would avoid even these chaste little pecks on the lips (though they are tolerable, and thus permissible) precisely out of fear of not being able to restrain my passion and subsequently falling into serious sin. St. Alphonsus Maria de Liguori, speaking about lust, says that just as straw placed near fire will also burn, so too, if two people not bound by the marriage bond engage in excessive familiarity, they will likely end up committing some mortal sin. Once a person is inflamed by passion, it is very difficult for them to restrain themselves.
As you told your son, not kissing his fiancée is an act of great respect toward her. It shows that he truly cares for her and that he is not with her purely out of physical attraction. If she does not understand these subtleties and takes offense, in my opinion, it is better to leave her, as it would be difficult to build something serious with someone like that. I would never marry a woman who does not want to live in perfect chastity during the engagement period. I would not trust such a person, and without trust, it is impossible to build a truly happy marriage. How could I be happy with a woman who is willing to trample on the Eternal Law established by God?
Kisses Between Spouses Are Not Sinful
For married couples, however, kisses, hugs, and similar expressions of affection are not only permitted but also useful in fostering mutual conjugal love. The crucial point is to prevent these actions from causing an involuntary emission—that is, the dispersion of semen, which may be deliberately effused only within the wife's reproductive tract and only during a marital act performed in a natural manner. (When the emission is involuntary, it is not sinful if it was caused by an action that possessed a certain utility, such as a doctor examining a beautiful girl, a confessor hearing a woman's confession, or someone taking a shower for reasons of hygiene, etc.). Chanson advises spouses to engage in hugs, passionate kisses, caresses, etc., especially before and after marital copulation, thus reviving the affection between the couple.
If you’d like to help your son, you could recommend that he read a book by Pierre Dufoyer for engaged and newlywed couples, which teaches many interesting things. I hope I’ve been of some use to you. Do not hesitate to contact me whenever you want to learn more about what traditional manuals of Moral Theology teach. For me, friendship means putting myself at the service of the person I consider a friend, with the aim of being of benefit to them—especially from a spiritual perspective.
I send you my warmest greetings in Jesus and Mary.
Cordialiter