Most Helpful Guy
Well if a guy didn't really care about boob size (or liked smaller ones), he probably wouldn't mind. But a guy that is dating a girl with DDs is probably very attracted to them and would be disappointed by a reduction. However, any guy worth having around would understand and want what's best for you.
Most Helpful Girl
Your health and well being is far more important than his sexual preference. If he doesn't like someone with smaller breasts he is free to leave and there's nothing wrong with that.
Side note: DD is pretty small in modern bra sizing www.brabandproject.com/gallery/search/cup/DD
It sounds like you are stuck with DD cups because stores near you refuse to carry the real range of sizes women need instead of just A-DD. For fun maybe try measuring yourself to see if there could be a more comfortable and supportive size out there for you. https://www.brasizecalculator.tk/