My child does not answer me in Arabic but I know he/she understands everything I say.
This is a very normal reaction! Our children feel awkward speaking a different language than the rest of the world he is living in. On the other hand, children are perfectionists, if he thinks his pronunciation is weak or if he does not find the correct word he will not even try.
One solution would be to make him feel he belongs to a special community; this is one of the reasons Alefb has been created. Show him how to live in Arabic not only speak Arabic. There are certain things you can express only in Arabic. Teach him all the new words in Arabic first.