Caught my son and his 'girlfriend' ?

caught my step son making out with his 'girlfriend' he lives with me even though his father has died... he does not call me mum but i do think of him as my son even though ive remarried ..any way earlier today i caught him and his girlfriend making out on the sofa and a condom on the table! he's only 13... i coughed and then the girl got of him and said 'cya later' and left.. and liam went up to his room ..what should i say to him.. ?