I only used a size one teat (Avent) with DD, right up until she stopped BF just before her first birthday then I switched to the size two.

If the teat is collapsing for your DS then yeah, you may need to change teats. I've had many friends have success with both Avent and Tommee Tippee bottles and teats so I think it's just a matter of finding out what works for your LO.

Sorry, that wasn't much help was it!