I'm applying for registration of my son, but I'm a bit confused on section 1.5

His name on birth certificate is: MATIAS MAY SORENSEN
Matias is his first name
MAY is his dad's surname
SORENSEN is mother's surname(mine)
So, How should I fill section 1.5/1.6/1.7
I'm confused 'cause 1.5 is asking just for surname
Thank you all!