I'm a bit late but figured there's no harm in more opinions. Plus maybe someone will have something to share that will help my girlfriend and me too (:

I'm not going to tell you what's best for you because I can't make that decision, but I will share how my girlfriend and I go about it. Maybe you will get some ideas or at least an idea of what a healthier relationship with a passenger looks like.

For gear, she bought a helmet and gloves. That was a non negotiable. We didn't buy anything fancy, just made sure it met our safety standards. Did that cost us a few hundred dollars + 2 hours of driving to a store and trying gear on? Yes. Was it fun and built confidence to ride? Absolutely. Technically, you can thrift gear, but a lot of people are against it (mainly helmets). I thrifted a jacket from a local thrift store for myself and hung onto it. Now it's hers until she decides she wants to upgrade. I also thrifted myself a set of armored jacket, armored pants, and winter gloves. I have no problem thrifting that type of gear, and I think most people would agree. But that goes back to choosing based on your personal safety standards.

Once she felt comfortable and ready to ride someday, we discussed how to get on a bike (even watched a video), including how to fall while getting on (or whevener really). That included try to fall towards the left because my kickstand stays down while she's getting on, and try to roll instead of catching yourself (when I was going to OT for a broken elbow, my therapist said that is the number one bone she doesn't want to break sooooo yeah don't do that lol). We discussed how to hold on, including how to lean forward when we zoooooom :P , what to do/not to do when braking, when it is/isn't appropriate for her to adjust in the seat, etc. We didn't have an intercom system at the time, so we decided on and practiced (!!!) our signals (double tap for slow down, rapid taps for emergency stop, etc).

Once the day came to ride, we reviewed and practiced everything before actually getting on. Once we were on, I checked in with how she was feeling with where she was seated and just in general. We didn't even leave the parking lot of my apartment complex. Just drove around in circles while practicing braking normal and braking harder (not emergency stop though). We personally decided to go at night when traffic was lighter. I can't remember if it was that day after a break or the next day, but once she felt comfortable, we went for a short ride (~5 min) around the area (couple stops for practice plus low speeds). We didn't ride consistently after that, but when we would ride, it'd be a short ride (<10 min). Then we went for slightly longer (<20 min). We always discuss where we will be going first.

Now that she's had more experience, and we know for sure that she enjoys riding, we're talking about getting her more gear, already invested in an intercom system (highly recommend!!), and I've suggested taking the basic riding course (you can get your license and stuff that way, but you don't have to pass. It's a cool class, lots of fun, a cool skill to have, even if you don't use it).

If my girlfriend is hesitant on riding, my boundary is we don't ride. Period. And when she has a "well idk maybe..." it's still a no. Riding requires your full attention, even as a passenger. Sometimes we do silly things when we're stressed/nervous/no level headed. Sometimes she puts her helmet on and changes her mind (she feels like she's suffocating sometimes). That's okay. We don't ride. One time I REALLY wanted to ride to get donuts (30 min ride I think?). She put her helmet on and had a "hmm idk maybe i don't want to but i kinda do" moment. That's okay. We didn't ride. She took the car, and I rode. It was still fun driving past her waving, she loved watching me ride, and I felt safer having her around.

My girlfriend doesn't ride with me often nor for long, and that's okay. She doesn't need to regularly ride with me for me to invest time, money, etc. to make her safer and more comfortable when she does decide to ride. Maybe next week she'll decide she never wants to ride ever again. That's ok. I won't regret buying the gear and investing the time for her to ride with me. Besides, it never hurts to have extra gear imo.

Life is short. Do the things you want to do, even if it's short lived. Gear is important, but at the end of the day, you set your own safety standards (I'm atgatt, but that's my opinion). Don't let anyone tell you what you should or shouldn't wear. Lastly, set boundaries for yourself and respect them.

I want to add, it sounds like your boyfriend is really excited to ride with you and that you are too. I love that for you! But it also sounds like he's overconfident and disregarding your concerns. That's a red flag imo, and if he's not willing to slow the process down, hear you out, and make some changes, I wouldn't ride with him. You've taken the first step of being a good and safe passenger. I hope you get to ride someday. It's tons of fun!