You can buy hard Avent spouts that will fit right into the bottles. They are for the Avent Magic Cups but all Avent stuff is interchangeable. They are always at chemists.

There are 3 types - a softer white silicone "starter" teat (not what you would want if he is biting through teats), a hard green one with small holes (minimum spillage) and then a completely open yellow and clear version.