I have a small penis, gynecomastia, always tired, retain a lot of liquid. Will go to an endocrinologist... What should I tell him and ask?

Everything. Tell him/her about your development during puberty. Any meds you take, drugs you use or used. Family history. Mention any other symptoms.
Just that question. . Endos deal with that kind of thing all day. Start with that and the endo will take it from there. They are very smart.