QuestionCan I get pregnant if only the head of the penis was in during his ejaculation?
Pregnancy occurs when a sperm reaches one of your eggs. A healthy male ejaculates about a half billion sperm each time he has sex. Each sperm has a tail allowing it to swim toward the egg. Then there is the fact that when an ejaculation occurs, there is often enough force to propel the semen six to twelve inches beyond the end of the penis. What all of this means is it doesn't matter if the guy only went part way in or if he didn't tear your hymen before he ejaculated. The chance that you could get pregnant is nearly the same as if he put his penis in all the way.
In addition your chance of catching a sexually transmitted disease from him is not different. Most diseases are passed by skin to skin contact, which you had or by the exchange of fluids and since you received his semen, you were exposed to any disease he might be carrying.
What you two were engaging in was fornication -- having sex without being married. It is a sin because it causes harm to those participating in it. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4). You need to stop sinning.