It is good that he is willing to play with his cousin and that he is receiving
speech therapy. Have you asked the therapist for his or her thoughts? They may know or have suspicions of a problem, but may not be allowed to bring it up.
Here in the US, you would either consult your pediatrician, a developmental pediatrician, or a pediatric neurologist. I strongly urge you to do so as soon as possible, as the earlier your son receives the necessary therapies, the better the prognosis is.