What can I do to help my boyfriend fix his papers if he is separated for 10 years and we have been together for 4 years?

The question is how is he in the US? Did he enter the US lawfully or unlawfully? Even if he entered lawfully and is eligible for adjustment of status based on a marriage to a US citizen, you first have to be married and in order for you to marry him, he needs to be legally divorced.

Much more information is required to properly address your concerns and hence, it is suggested that you and your boyfriend consult with a well recommended immigration attorney in your area. But I can say this if your boyfriend is presently in the US, one way to help him stay in the country will be to file a Form I-130 petition for him, after your marriage and then, assuming you are a USA citizen. Given the paucity of information you have provided and realizing that there is a checklist of several issues that you must both address if he wishes to obtain a green card based on marriage, I find that your best bet will be to sit down first with an experienced immigration lawyer.

I am assuming that when you say "separated", you mean that he is married but not living with his wife. Until he is divorced, there is nothing you can do.