Maria Montessori anticipated negative comments about children using fragile, breakable items in her Children's Houses and as such she wrote:

...they place more importance on the glass than on the child; an object worth a few cents seems more precious than the physical training of their children. - Maria Montessori. The Child in the Family. 

As we know the importance of sameness, I would recommend purchasing 3-6 weaning glasses of the same type. Unfortunately I did not do this and now Otis is having to use an unfamiliar glass, for the second time. In one month we have had two breakages, many near misses. We have tile flooring and every glass that hits the floor will break.

My thinking has waivered on occasion. Is he too young? Am I expecting too much? Perhaps we should wait?

I have concluded that it is too late to go back. If I give him an unbreakable glass now, the previous lessons will be wasted. Is he too young? I don't think he necessarily understands everything that is going on around him but he is absorbing it.

Do I need to be more attentive when he is using his glass? - Yes.

Will there be more breakages? - No doubt. 

I do need to remember that Caspar who is now four, didn't use a sippy cup but started with plastic cups, still broke glasses after they were introduced to him. He has broken 3-4 in his life, usually when he is being silly (do not hold the glass between your knees or balance it on your head!). Perhaps if Otis learns now there will not be these breakages when he is older.