hi meg,

i would say, that this is to early to sit for a baby. the milestone for sitting and crawling is 9 month. your baby is definetly to young for such active pos. you see, the baby has to keep the pos. on its own, without any helping advices. passive sitting puts to much load on the spine. your baby should be able to lift up on elbows and hold the head on its own. the trunk rotation should start and maybe sometimes the baby presses up on hands.(milestone for 3 month old babies). but i am sure that it is not strong enough to sit properly and free on its own. but as i told you, the baby likes the position because its easier to see the environment. for a few min its ok, but to long will weaken core muscles and development of the stabilising system.
cheers