There's almost no possible way you could be pregnant. But, pre semen can have some sperm in it. Especially if he masturbated within a few hours of touching you.
Don't take any chances. Before you let him touch you make sure he washes his hands with soap and water. There should be a period of time in between when he touches his self and touches you to make sure any sperm will die. It dies pretty fast outside of a warm moist environment.Also, if his hands are not clean you could get a bacterial infection.
If you two are at this point things can get very heated so I recommend you think about getting on birth control. You can get pregnant your first time of having intercourse so you need to be on the pill, have the implants, or and IUD before having intercourse.
The cramps and back pain was a good sign. It meant that your period was most likely on it's way.