I have a 13 year old son who is small like an 18 month old infant. He cannot sit on a normal chair. Do the Shinkansen have seatbelts like on airplanes, so we can attach an secure the infant car seat. Otherwise, are there places where he can sit in his wheelchair and still be with his family.
Shinkansen seats don’t generally have seat belts. The ride is very smooth so there is very little worry of falling out of your seat. Most shinkansen do have wheelchair accessibility, with several spaces where a wheelchair user can sit with their family. The Hokuriku Shinkansen E7 series Kagayaki, Hakutaka, and Asama trains have several wheelchair spaces available. You may need to book your train tickets a few days in advance to ensure a timely departure. Please speak to a teller at the Midori no Madoguchi in a JR Shinkansen station upon arrival.
Also, see this article on boarding the shinkansen as a wheelchair user for more information.
I hope that helps answer your question.