Partner ejaculating in WOT

My partner and I have a healthy sex life and he can ejaculate in just about any sex position that we're in, except girl on top. While he enjoys the view of me riding him, he rarely comes inside me in that position. Once we change positions to almost any other position, especially missionary and doggy, he ejaculates fairly quickly. My guess is that my vagina is not as snug for his penis as the other positions. How can I stimulate him more when I'm riding him? He says my vagina is plenty tight and it's not an issue with other positions.