I live in Oregon and am in a relationship with a man who has a 15 yr old son from a previous relationship. The child's mother is deceased and I would like to legally adopt him. Is this possible considering we are not married and do not plan to be.

Depends on your location, in the US most states require marriage. I would even personally question it, since if there is no intent to show a seriousness of the relationship and to show it is stable by marriage, why would you want to adopt.

Next at 15 what does the child think about it, He would have to want it and agree also in most cases at such a older age

Your boyfriend would have to give him up, relinquish all his parental rights, in order for you to adopt, since you two aren't married. Why in the world do you want this? And it takes time. When he's 18 he's on his own.

Oregon's Domestic Partner law does permit "step parent" adoption by registered domestic partners. However, I don't now if you qualify.

I suggest you contact a local Family Law attorney who is familiar with Oregon's laws.