He wants to have AGAIN...should I say no?

My close guy friend is 17, and I am 21. He will be graduating as valedictorian next week from high school, and he told me he didn't want to graduate as a virgin. He knew I've been around the block a few times, and I reluctantly agreed...and we did it. I was on top, and it obviously did not last very long, but we had fun. Now he's constantly pestering me for more...which is surprising because he's very insistent that his parents don't find out -- especially his mother, who entrusted me with looking after him and being an older sister figure to him. I told him that it was an one time only thing - that he should think of it as an early graduation gift, but I worry I may have messed him up...it did feel wrong then and it feels wrong then. I honestly don't know what to do. I'm not sure if I can stand having with this boy again without crying because he's really too young to be even doing this stuff...and it was the only time I ever had with a younger guy - and making it worse, a close, younger brother like friend.