I do think that at 10 months he may be a bit young to drop to just 1 bottle, but you know your lo better than anyone else.
I would keep an eye on him at this stage, once the ulcer has healed and the teething has stopped for a bit, see how he is with the 2 bottles again.
Both the calcium and liquid intake are equally important. If your lo is not taking much milk, I would make sure you up the calcium intake with cheese, yogurts, milk in cooking and cereal as well as custards etc.
I would ensure that you have a sippy of juice / water (whichever your lo drinks) available at all times to ensure that your lo is drinking enough.
HTH's
Paula