The ladies have done a terrific job of encouraging you to keep going with the hymns! I agree that the theology and stories behind the hymns are inspiring and such a testament to the faith of the writer. I'd hate to have your son miss out on knowing these wonderful hymns. Plus, we've found no more beautiful orchestration combined with singable voices than the ones in the Hymns for a Kids' Heart series.
With that in mind, a few things that have helped my boys sing along are for all us to sing the hymn together. I'm not sure if you have other children, but we had all of our kiddos (and me included) sing the hymn right before we ate lunch each day. I photocopied the hymn for the week and gave everyone a copy to sing along. I played the music VERY LOUDLY so that you almost could not hear my kiddos' voices. Then, we all sang together around the kitchen table. If they weren't singing or weren't singing joyously, we paused the music and began again (thus delaying lunch). A few times, we restarted 4 or 5 times in one day. The boys got the picture pretty quickly that if you want to eat, you sing first.

Over time the habit of singing joyously and loudly actually transferred to true joy in singing. It took time, but my kids love and remember these hymns even today when they come up in church. They all look at each other and just sing their hearts out.
If you don't have other kiddos to join in the singing, you could instead do it before the evening meal and get dad involved in the singing. When my husband sings with the boys, they really join in. There is just something about singing in a group that makes you feel less intimidated as you sing.
Anyway, hope this helps!
Blessings,
Carrie