I'm the one with diabetes but my girlfriend is in a similar situation as you. We are 27 and 28 years old so close to you.
I'll share the things I love most about what she does for me. Of note: I was just diagnosed with type 1 a couple months ago.
-we flew for the first time last weekend. She asked airport security for any advice about traveling with my insulin/needles/etc.
-when I have a hypo(low blood sugar) at night she will go get me orange juice. She also keeps glucose tablets in her purse just in case.
-she sometimes comes to my doctors appointments so she can ask questions and learn more too.
-the day I was diagnosed, she was in New York on business. When my mom called her and told her I was in the emergency room she took the first plan home to be with me.
Ultimately, she's always there to listen and offer to help but she never babies me or acts like my mother would. The only thing that frustrates her is that I don't drink anymore so now she has to drink alone when we go out to dinner. Of course I'm (half) joking in saying that.
Side note: it's a shame your boyfriend tries to hide his diabetes. Hopefully he doesn't feel ashamed. I personally don't hide mine one bit.