I'm 28 yrs old and looking for my biological father. I was told that the man who could be my dad passed away in 2002. What can I do?

Very little I'm afraid.
Are you 100% sure of these facts ?

Yes , I'm 100% positive

Does he have any other children? If yes, would it be possible to have some sort of DNA test compared to them to see if there is a relation?

Yes you can contact his sons or daughters and while not as perfect as having his DNA, it will be good at letting you know if it is true or not.