Hi rowberryhatton,

I agree with Esther, that at that age standing is done through reflex activity, which I don't think should be encouraged. In relation with the baby bouncer, the epifisis of the bone (cartilage near the joint - sorry if misspelled..) is stimulated to grow on a particular area (medial or lateral) depending of the forces that are applied to it during early weight bearing (in crawling, kneeling, standing...), hence that babies have a physiological valgus first and corrects later. I'm not sure the bouncer is the most recommendable for this joint development. Appart from the irregular force on the bone, the hip joint is held on an abducted position.
Again, if you do it only for short periods mightn't be as bad, and he is gaining other sensory experiences. Still, if I had to give my professional advice I wouldn't recomend it or use it for my baby.
If you would like more information on developmental orthopeadics, pathways web page from Beverly Cusick is a good source of info. Do a search in google and it should come up.
Hope this helps. All the best.

Eva