You have to experiment with angles, how he strokes, positions and dynamics of foreplay.

Most women don't cum from penetration though, don't matter the size and the headspace. You could be one of them.

However if you are one of who can actually cum and what's missing is feeling more, something more intense and more full, you are basically screwed. His penis is insufficient and you both can't change that. He will be disabled in that aspect. You can easily find out this with bigger toys.

There is a possibility too that you might not cum from penetration, but still enjoy a more fuller and intense sensation. In that case he is also disabled to do so. You can resort to toys, but the real thing will never be possible unless you experiment with non conventional sexual dynamics, like open sexual relationships, cuckold or weird stuff like that.