Keep signing, and keep trying. Here is a study that indicates that your son, even though his ASL input is inconsistent, will surpass his models at home in proficiency. As a deaf child, he has an innate understanding of the morphemes and grammatical structures within a visual language. The study is, When learners surpass their models:The language acquisition of American Sign Language frim inconsistent output, J. Singleton & E. Newport. It is available online at
ScienceDirect - Home, or fromthe journal of cognitive spychology 49 (2004) 370-404.
And, yes, he is just a kid, and needs time to just be a kid. Lessons at home need to be very subtle, not formalized like school. Just constant input and interaction.
If you can, find a deaf club or organization close to you and begin exposure to the Deaf community. You will be amazed at how quickly his ASL skill explode.