Identify this 70's docudrama

I recently remembered this movie from the early 70’s that, around 1990, was regularly broadcast on Discover Channel. It was about a mentally retarded boy and his daily life. Although it was a dramatic film, the naturalistic shooting style made it seem like a documentary. The only scene I clearly recall was him going to the record store to buy albums (I believe the Carpenters and Jerry Reed). Anyone remember this flick?

Could it be Best Boy?

Nope. The boy in the movie was literally a boy, at most 18. Plus there weren’t really any changes in his life during the film, it mostly seemed a slice of life of his everyday actions.

Hmm…maybe the TV movies

And Your Name Is Jonah (1979)
Like Normal People (1979)