Iknow a 15 year old that is dating a 21 year old, they have been together for 3 and half months and love each other. Could he go to jail for being with her?

If he is having sex with her, very likely. If her parents don't approve and forbid, also likely.

Since there are no laws governing dating it depends on those factors.

However, I have grave doubts about a 21 yr old involved with a 15 yr old. There is a very wide gap in development here. I can't imagine what a 21 yr old would find in common with 15 yr old. So either the 21 yr old is developmentally challenged or is grooming the 15 yr old for sex.

If you are in Toledo, Ohio, he could be charged with statutory rape if he has sex with her:

"... 2907.04 Unlawful sexual conduct with minor. (A) No person who is eighteen years of age or older shall engage in sexual conduct with another, who is not the spouse of the offender, when the offender knows the other person is thirteen years of age or older but less than sixteen years of age, or the offender is reckless in that regard. ..." Ages of consent in North America - Wikipedia, the free encyclopedia

If you are in Toledo, Spain (or some other Toledo), the law might be different.