One of my very close friends growing up came home early from his mission, entirely because he didn't feel right telling people they could have experiences he hadn't had. He asked for the release, and he was given an honorable release by a good Mission President who understood. I admired my friend immensely for that, and even more so when his girlfriend broke up with him because, in her words, she only wanted to marry a "real" RM. (I really loathe that attitude, and I choose that word intentionally.) He faced some judgment from others when he returned home, but his parents were very supportive - and he has a strong testimony of compassion, love and charity as a result of his experiences.
I would share with your son the details of Pres. George Albert Smith's life - of his constant physical trials and his struggles with depression throughout his life. I would explain that Pres. Smith was one of the most loving, humble, charitable leaders we have had in the Church - and I believe his struggles were a large part of reinforcing his natural tendencies in that regard. If a President of the Church struggled as much as he did, there is NO shame or guilt whatsoever in your son's situation.
I also would have him read 2 Nephi 4 from the perspective of a clinical evaluation of Nephi. I believe there is a very good chance Nephi would have been diagnosed with depression, bi-polar disorder or something similar, based on the symptoms in that chapter. That chapter is one of the reasons he is one of my favorite prophets - not because of his recorded visions. I appreciate the struggle described in that chapter FAR more than any sanitized accounts of other prophets.
More than anything else, I simply pray for him and your family. May you be able to rise above the pettiness that you might face and be the example for others they should be for you - amid your struggles, not without them.
One of the most oft quoted verses in the Book of Mormon is 2 Nephi 25:23. It says:
"we know that it is by grace that we are saved, after all we can do."
I'm NOT going to go into the different ways that verse can be interpreted right now, but I think it might help your son in his current situation to understand that, no matter what others think about him and his release (even the leaders of your ward and stake, perhaps), and no matter how the verse above is interpreted, it says in crystal clear terms that your son has been saved by the grace of God in the specific instance of his missionary service.
Saved from what? I think it is fair to say from every negative aspect of his release. Period.
Isaiah 53:4-5 says:
Surely he hath borne our griefs, and carried our sorrows . . . the chastisement of our peace was upon him; and with his stripes we are healed.
The parts I skipped don't apply to your son's situation, but the parts above do. I hope he can recognize what is taught in these three verses and accept that, in God's eyes, his service was enough - since it was "all (he) can do".