Though he can hold his position long enough for you to snap a photo, your little bundle still topples over like somebody spiked his bottle after a few minutes of sitting up. What's going on? Holding yourself upright independently is harder than it looks -- it requires muscle development in the upper back and neck, as well as balance. While there's no need to rush this milestone (some babies don't sit well until 9 months and that's fine), you can help him along with this gentle workout:
Watch his back. Any couch potato will tell you that sitting is supposed to be relaxing. A Boppy or other low pillow to fall on can help him feel more secure.
Keep sessions short. Usually it's muscle fatigue that knocks your baby down. Limit sitting practice to several short periods so he won't get too tired out.
Boost his balance. Reaching from a seated position is the next step, so entice him by playing a clapping game, showing him a toy, or rolling a soft ball toward him.