he isn't getting enough stimulation during sex.you need to try other positions.
maybe I should clearify,He takes Viagra when we have sex because he is unable to keep an erection for a long time, but does very well during oral sex.
the oral sex gives him more pleasure and that is why he can keep his erection.different sexual positions should help this.
try these.
http://www.associatedcontent.com/article/304634/6_penetrating_positions_for_a_small.html
Thank you I will check out the link.