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(I like your moniker, BTW!): Great question. In general, young romance tends to be intense... but it sounds as though your son may be bordering upon too intense - and in a relationship which (sadly) will probably end sooner or later. My biggest concern for him (frankly) is how he will handle the break up when it does happen if this intensity remains at the current level.
But a couple of things to think about: (a) the intensity of the relationship may be driven higher by the general state of anxiety he is in. The energy created by the anxiety may be pushing on the natural energy of his first love (plus the hormonal craziness that is surely going on). Treating the anxiety may help the relationship to relax as well. And (b) if he has experienced loss in his developmental years (divorce, death, abandonment), then those old scars may be getting ripped oped by the new intimacy, and may be driving the angst. Again, treating the loss and subsequent feelings may help him to create better boundaries and realistic expectations.
In the short run, monitor not only his emotional state, but any signs that he may be overly jealous or controlling with the g.f. We wouldn't want the angsty energy to create a toxic relationship, especially if she has male friends, etc.
So, talk to your son. Let him know your concerns and that you are invested in making sure he has a full life that is rounded with other interests beside the g.f. He is not forbidden from seeing her or texting, chatting, etc... but you need some evidence that he is tending to the other matters that are important (studies, college apps., job, etc.). When his life has taken on all of the roles that are normal for a kiddo his age, then you will feel better and not be adding your own anxiety into the equation.
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