If your son is 19 yol, his feelings may be due as a result of transference
The feelings are not per se "abnormal" and they are not either the "norm"
It would depend on what issues he was working on in his therapy
If she was helping him overcome anxiety, then his anxiety may have manifested now because he's become too dependent on her to help him manage it
He could be making an association b/w her availability to conduct his sessions and his decrease in anxiety. Now that she's away, he may be mistakenly allocating his inability to control his own anxiety as a result of her absence (rather than his own control over it)
Abandonment issues is another thing that an older individual would not feel unless he's developed other feelings (amorous ones) towards her
If her mere absence is causing his anxiety to increase, it is an indication that there is underlined unresolved issues
Even if this is more of a codependent versus healing/therapeutic relationship
If you're still online, feel free to reply.
His anxiety may be due to possibly worrying (how she would deal w/ her own crisis) as well as the delay of getting feedback from her in regard to his texts. Yet, no client should be so reliant onto another (even therapist) for alleviation of one's symptoms. So, to answer your inquiry "Is this a normal reaction" the reaction is not the norm for an adult 19 yol to display. I would refrain from the word "normal" as surely he's dealing with issues that she is helping him with. That does not imply abnormality; simply that he needs help w/ it.