When the mother pushed them Sone protect Masa and ended up with a more dangerous impact on the fall.
No but 3 years had passed....and in the school and home???
thats why, Sone ended up in a coma. The mother didn't care about him so just left him there, but Sone's subconscious reached out for Masai so he could finally remember Sone and help him wake up from the coma
Look carefully at the scene of the last panel. Near Sone's head flows blood. So he had a blow on his head and thus fell into a coma for 3 years.
Masai only lost his memory 'cos Sone probably protected him from the fall
http://www.mangago.zone/read-manga/nani_mo_iwanai/uu/de937c1e73616f42a09758e2df5d9d5a_chapter-3/pg-35/
Why was sone in the hospital???