A student forgot to answer an exam question
You should give zero for the answers in question. You are grading what the student handed in, not what s/he potentially could have handed in. The logistics are just as much part of fulfilling an assignment as the knowledge. It would be unfair to the other students to be more lenient on this particular student than others.

If you wish, you may give the student another chance to answer these questions, but then you should do the same for all students.

(I once failed to notice in an exam that the question sheet was printed front and back, and only answered the questions on the front. I duly accepted my C+, along with the two others who had made the same mistake...)