Hey Wendy, just butting in here
I def wouldn't worry too much about the fact he is 12m and being off bottles, I think it is quite rare that they are off them quite that quickly, despite advice. DD was on the sippy by 15m and I was thrilled with that. How adaptable is he where food is concerned? I was lucky with DD and was able to just completely switch to cow's milk when she turned 12m (in a bottle) and she didn't pay the blindest bit of notice, never went back to formula then. But of course you could always try replacing some formula with cow's milk and going that route.
And re swapping to sippies, it was a bog standard sippy that worked for us in the end as she only took a straw cup for a day or 2 before rejecting it. And I took the quite firm standpoint of milk in a sippy or no milk at all (not that I was standing in front of her shouting that
I just only offered the sippy and let her drink what she wanted, didn't revert back to bottles after a couple of days of doing that and her quickly figuring she didn't even need to try the sippy as mum would just put it in a bottle anyway!) So anyway after a day or 2 she just accepted the sippy and we never went back, it was so easy in the end, I feel very lucky! But like I say she was nearer 14/15m and like you I had no concerns about her milk intake as she is such a big eater. If the worst had come to the worst I was just going to increase her calcium intake in all other foods throughout the day and add more milk to things etc.
HTH and you don't mind my tuppenies worth