Question: Our 20-year-old son recently returned home from a year learning in Israel. He is boy who struggled a lot in his teens both academically and religiously. Baruch Hashem, he seems to have had an excellent year of growth, he likes his Rabbiem and has learned a lot about being a mentsch , though he still has a way to go. He is very proud of his year and some new concepts and ideas he has learned. The yeshiva has an emphasis on emotional health and he has really taken to it. He has however become very opinionated and not very flexible and can dominate conversations with his ideas that he considers to be 100 percent true. He has long speeches about the problems with the "system" and has all sorts of recommendations for his siblings (ages 9-16) including that they should all be in therapy because it is good for everyone and he even suggested one of his sisters should try medication because "it's not a big deal." He can also overshare a bit. We have talked to him about it but he has become a bit of a bal shita about things. This is better than the unhappy kid he used to be but this has become uncomfortable for us and his siblings. How can we continue to be supportive but encourage him to be a bit more open to the fact that not everyone shares his experience and ideas and not all ways are apropriate for everyone?
One can approach this question and attempt to understand the core issue from several angles. While there are certainly specific personality variables that play a role, I prefer to tackle it from a developmental approach and explain this phenomenon through that lens. 
Adolescence and early adulthood are developmental periods suffused with idealism.  Witness the social movements and upheavals that begin on campus. In our circles, Yeshivas and seminaries capitalize on this youthful idealism and channel it in the direction of their specific cultural value (e.g., Limud Hatorah, Chassidus, Aliya to Israel). This period is one where perfection & shleimus are emphasized and channeled. 
Transitioning from this phase into middle adulthood usually involves a "landing" process which most can successfully navigate. Most can retain their core values, while tempering and nuancing them somewhat. As she experiences "real" life, the arch-liberal college alumna begins to understand a conservative outlook. The bochur who was destined to learn in Kollel for life recognizes the reality and begins taking some courses while maintaining his Torah hashkofa and Sedorim. This is normal development. For an example of this, seforim note the difference in approach between Rav Shimon Bar Yochai and his son, Rav Elazar (Shabbos 33b).
Those entering this phase with mental health and personality struggles will inevitably have a harder time with this transition. This may be true with your son. He may struggle with perfectionism, rigid thinking, and unrealistic expectations, which may interfere with this transition. Ideally, yeshivos and seminaries should flag these individuals and ensure that throughout they receive a "grounded" message.
The task for you right now is twofold. First, appreciate where he is developmentally and understand that much of this will gradually cool with the passage of time. A second task for you to consider is to introduce him to a professional or mentor whom he can hopefully respect and can help shift his perspective. Within the right relationship, these individuals often seek religious grounding and nuance. They seek to idealize someone who has successfully navigated this murky process and look toward a healthy role model as a template to work off.