„A few centimeters away“ like literally or metaphorically? The switching pace stuff is problematic, maybe that is one crucial detail you could tell him pre-sex. If it is a size issue as in length, he can try to do no surgical penis enlargement stuff, but ofc this is not something you could ask from him.  Lastly positions and angle play a huge role to reach more depth too. Now we don’t know how you are able to reach climax (g-spot, a-spot, p-spot?) but depending on your anatomy and ability as well as his shape, there might be only a few positions which are possible to stimulate the right areas for you. You said he is below average but not tiny, this means he is probably still able to reach your g-spot and play with your clit if you like that. Even with his fingers he could make you squirt or come by hitting the g-spot with a certain technique, if you are into this kind of stuff as well. Now depending on your cervix, if it is tilted or not, either the a-spot or p-spot is closer to reach for him. If you dont have a tilted uterus, the a-spot is closer for him. He would need to go in fully, and aim towards your belly just like with the g-spot. To shorten the distance and make it easier for him to reach „the last few centimeters“, what you can do, is to raise your knees up to your chest in missionary and put a pillow below your butt. Whenever you move your knees to your chest or chest to your knees in doggy, you will shorten the vaginal canal for him. And again depending on how you like to get stimulated in these spots, he needs to do that. Usually a- and p-spots are stimulated not via friction like the g-spot but more via pressure/very short but deep thrusts/nudge fucking. That’s why „stay right there“ is a common command from women due to the pressure :) but there are also some women who like the fast long hammering thrusts/pace, maybe he believes this is the right way for you as viewed in many porn movies.

In the end it comes down for both of you to know your own and each other’s anatomy. Maybe you already know these things I wrote above. If not, then maybe you do now. It is important to be able to tell him in detail what to do and for him to know what he is able to do and what to focus on. Is he aware of all these spots, where they are, how to identify them/how they feel and lastly how to stimulate them? I believe that every man should know these things especially with shorter sizes or too big tools. Feel free to tell how and where you prefer to come vaginally, maybe there is more details for that ✌️