Hm, it seems that person who took your son out of the Deaf school and into mainstream has no clue about Deaf education apparently. Ok, Deaf school and mainstream is somewhat equally as the only difference between both is that the Deaf school teaches using Deaf native language which is American Sign Language and the mainstream doesn't. In some Deaf schools you can go, you can learn same thing from mainstream school that you would learn from the Deaf school except that you would learn through ASL and mainstream often is through spoken language.
Mainstream doesn't offer what Deaf schools offer and that's Bilingual-Bicultural (Bi-Bi) program. Your son would be bilingualism which is something that you can rarely get from any mainstream. Deaf school teaches you ASL and English separately. So two languages instead of just one language like many mainstream schools only teach and bilingualism is beneficial. Who doesn't want bilingualism in their resume, business loves hiring people who has bilingualism skills. :P
That's just my second cents. I suggest that you contact your state association of the Deaf and ask some help which I am sure that they will help you. I am Hard of Hearing myself, I have went through speech therapy and all that, wanting me to be oralism but unfortunately that never worked and decided that I only learn American Sign Language I succeed to college level. I am bilingualism.
The bottom line, if your son is Hard of Hearing or Deaf. As in early stage, you might want to focus on having your son to receive education using American Sign Language instead of spoken language or "hearing school". I went through it and I sued board of education for that very specific reason because I would've learned a lot of more if I was to be taught using American Sign Language instead of struggling to lip-reading teachers spoken language.