First of all, I feel your concern for your child. Whether or not a special needs child is able to blend into the society depends on a range of factors, such as his learning ability, the educator, the allied educator (speech therapists, occupational therapists, etc), parental guidance, peer relationships, learning environment, and many more.
It will be good if the SPED school has the range of professionals (as mentioned above) to help to adjust your child's learning style and help him with his skills/learning competency as such. However, it will be even better if your child is continuing his studies in the mainstream pri school, together with classes with the allied educators and special needs education (be it school or tuition).
Just a small query, what do you mean by your other concern of being lagged far behind? In which area are you comparing with?