Gentle prodding.
"Hey, we're going out to hike a trail. It's only for the morning. Please come along with us."
"Dad's going to the baseball but has no one to go with him. Can you go too so he doesn't get lost?"
"Hey, we're going out to hike a trail. It's only for the morning. Please come along with us."
"Dad's going to the baseball but has no one to go with him. Can you go too so he doesn't get lost?"
In the end, the genesis of Hikkikomori's reclusion is their feeling of having failed at life. They can't deal with society's expectations of them, so they withdraw from it completely. There's not much parents can do to change that feeling of failure.
It's not really a distinctly Japanese phenomenon. I'm sure you can find just as many young recluses in every developed country. Japan just found a cute term for it, that's all.