When you say functional language what do you mean? Words he says are they repetitions ? How is his eye contact? Any repetitive behaviour?When did he start speaking sounds or ma or baba? A complete evaluation is important. Take him to a developmental paediatrician or a child psychiatrist for diagnosis.
Next Steps
Iq testing, hearing testing, ADOS,ADI-R tests should be done
Health Tips
Speech therapy & occupational therapy help in most cases with developmental delay.