SSRI medications like Lexapro, Zoloft, Prozac, etc. are helpful for many people. BUT for others they can cause or exacerbate negative and sometimes harmful behavior which slowly appears, or more suddenly arises especially in the earlier weeks/months of taking meds.
Do a little basic research by googling "SSRI drugs and side effects." Studies in Germany in the 1980s showed that some young people in particular, who took certain SSRI drugs became aggressive, hostile and experienced suicidal ideation. Very problematic issues.
Other research reports have come out showing how negative side effects can also occur sometimes in ADULTS, not just juveniles. In the UK, Paxil was banned or restricted like another similar drug (Celexa?) I believe. A friend of ours went through this and it was a mess for a while, until she got to a good doctor and on a healthy course.
Since 1990s SSRI meds were tremendously popular and overprescribed, for anxiety and depression to nail baiting, eating disorders, you name it. A lot of money was made off them, and sometimes they caused problems, bad outcomes. Yet for many people the meds were, and are beneficial. During the 'Prozac Nation' years, stressed out teenagers, post-Iraq Vets and many other people were often started on meds without serious, professional evaluation and MDs requiring regular medication checks and monitoring visits every few months which are critical.
Not trying to scare but inform you, like other posters who've stressed the importance and urgency of having your son consult a MI medical expert, not a General Doctor.
Also I'm NOT anti-medication. You said your son is going off Lexapro after recently starting it. The behavior you're describing, notably the 'personality change' and resistance, and the symptoms and diagnosis that initially warranted medication definitely must be evaluated and regularly overseen by a very competent psychiatrist/medical expert. *Mention family history, substance abuse, any other medical conditions and ALL medications taken, including OTC (over-the-counter) products like decongestants, supplements, etc. It's important because some people are sensitive to drugs of all kinds, especially if mixed together or combined with other substances.
That your son is a kind, lovely man I'm sure. A neighbor with a different condition is like that- sweetest guy, kind and giving to all, and much loved. Aloha, all the best to you & your son, help is out there!