Thank you for your query.
can be corrected by speech therapy
and a little perseverance. It is better to work with a speech therapist in your area.
As you have not mentioned the age of the your son, it is difficult to provide some guidelines. A toddler to 5 year age need not be pushed too much. However, if the child is between 5-10 years of age then do work with the speech therapist.
I am not aware of any speech therapist in your area and hence cannot be of much help there.
I hope I have answered your query to your satisfaction.