It is possible that right now your metabolism really is so fast that keeping weight on will be really hard, but you won't know until you really try

And even if you can't keep it on now....a lot of people unintentionally gain weight not long past being 18, I really think that is a big part of the 'freshman 15" simply that people hit that age where they can't just keep eating junk without gaining. So keep at it, and if you don't gain right now, you might find that in six months or a year suddenly you do start filling out.
If you are frustrated with waiting for the weight, some things you two can do to have some weight related fun in the meantime:
- obviously go out and get as stuffed as possible sometimes, which will at least temporarily round out your gut, and tell your boyfriend how full you are and how big your belly feels and how much you like that and want it to grow.
- try doing some padding. Buy some cheap clothes up a size or two, and you can start with something as simple as small pillows or towels, filling up your clothes to look a little bigger. Again, then talk about it, how you can't wait to be that size, get him to tell you how hot he'll think you'll look, etc. If you are more daring, go and try on clothes while padded a bit.
- plan together how you can eat constantly. That is, go through your schedule for the next day and plan out all the food you'll eat, figuring out all the opportunities. Like "OK, so I'll have breakfast at home like normal, then what next? Oh, there is a donut store on the way to class, I could manage a couple of donuts I'm sure. And then between classes I could have a muffin, if I brought one with me. Oh, and the vendor machine has milk, I could have a carton of chocolate milk. Soon after that it will be lunch, and....."
As for how to change things so you can actually gain, everyone is different, and probably it will take some experiments to find out what works for you. I'd try that suggestions of eating all day long. Where your boyfriend may be able to help you is by reminding you to eat. I don't know if you are together at all during the day, if so he could even give food to you. But if not, he can help make sure that you have snacks with you, and then send you text messages or something to remind you to eat something.
Also, there are different kinds of feedee and feeder. Do you want to be made to eat, do you just want praise and encouragement? Does he want to be commanding and in control, or working with you cooperatively, or just watching and enjoying? Something for the two of you to work out between you.
Good luck, and have fun!