Sexual activity is involved

Depends on where,

In the US and Canada and China there are no laws on dating ages. There are ages where they can have sex. * which I will guess the 25 year old will want for normal 25 year olds**

For the 17 year old, if the parents of the 17 year old, order the 25 year old to stay away from the minor child, in some US states the 25 year old could be charged with custody interference

In most of the US by 17 you have reached the legal age of consent, but there are many places where it is still 18.

As noted the laws don't address dating, but a 17 yr old is still a minor and parents can order the 25 yr old to stay away.

How long has this dating been going on? How long has it been sexual?

How long have you been involved sexually with this man?
It depends on where you live, but you are still a minor and your parents could have a problem with this and take action against him. I know as a parent I would be trying to find out why he is seeing you and not someone his own age. Are you still in High School?