Assigning the correct house

1
Would like advice please on which house to assign for the following. I am asking the question regarding my grandson. I assume that the house would be the 9th, my son's son. His son is actually his step-son so would that make a difference as I assume that we would use the 3rd house - my son's wife child. The question relates to my grandsons work experience at a company with the possibility of future employment there. Which house would I use in this situation. Thank you. Happy bank holiday.