ok so he just got stab and run a huge fever and... he is now well enough for sex... really

i believe there was a few months in between. Since in the beginning of ch. 5 it was fall and I believe he got stabbed in the beginning of the summer when he went to visit. So it seems like theres a 3 month time gap

No the first time they had sex was just days after he had been stabbed. He was still bandaged. He went home and applied for school in Tokyo, so the second time was months later