The phrase "slow and steady wins the race" is key here I would say a little patience, a nice lubricant, and either a few of his fingers (or a toy) are going to be a good starting point. He can easily use his fingers (one at a time) to work you into a more accommodating size. As you are able to accept 1-2-3 fingers comfortably you can either graduate to a nice dildo or choose to try him on for size. Once you start taking him (and it may be at a very slow pace for the first couple times) a few times you will start to accept his size more easily. Trying different positions may help also.

My girlfriend is very petite and I'm reasonably well endowed so had similar issues starting out. It didn't help that I'm the first person she's dated or had sex with, so in addition to starting slow there was aspects of nerves for her in play.

I spent nearly a month slowly starting with massaging her to help her relax initially, eventually moving to teasing her clit, and then moving to going down on her to start. I then progressed after a time to using a single finger, eventually moving adding another finger after about a week with one. When we got to the point she could take 3 fingers and enjoy the experience we moved to her trying to take me PIV. Had some issues with her vagina clamping from nerves at this point so took another two weeks and talking over her concerns, nerves, etc before we eventually succeeded; however, now we have no issues with me getting in with minimal foreplay and a bit of lube. We need the lube more due to she doesn't get as wet now that she's on the pill, as that's seemingly a known side-effect.

But take it slow, use lube, relax, and most importantly you should both enjoy the process.

The human body is rather amazing. It is very resilient with multiple redundancies.

Strangely enough no amount of big cock will permanently stretch you out. Gynecologists have proven that the vagina will return to it's original size after sex even with exceptionally large toys etc.

However after popping out a baby the muscle fibres break and some elasticity is lost.

But for general principles the vagina will not stretch permanently. That said the muscle memory gained from repeated sex with large items will allow your body more comfort.

Your body learns to accept the larger objects and in time your body will automatically relax, produce additional fluids for lubrication and if need, trigger orgasm earlier to release endorphins etc.

The human body is amazing.

There are dilators to help with this, and clinicians to train ladies how to stretch for their man. My wife is a pelvic floor therapist, and deals with this all the time.

There are other options... some extreme options...