Mental retardation is not a dieasese ,but it is developmental delay .For example a 10 year boy may not be behaving age appropriate or he may not be intellectually functioning age  appropriate.This should be assessed by a clinical psychologist to know the degree of retardation.Then the  required intervention.Consult me or any clinical psychologist for further help and advice.

Whats his level of relationship mild moderate severe how much is the IQ?

Firstly what is the level of intellectual disability? Also what test has been done to determine this? Would be easier to answer once these are answered