I, too, would suggest that you see the vet as soon as you can, however, the two events may not be related... He may have hurt his foot otherwise, not food related.
I would also recommend that you look at the food section on this website, there is some great advise and while I understand that you may not have access to ALL the brands, you should be able to get something better than Royal Canine.
Also, was the food switch gradual or sudden?? If it was sudden, that would explain why he did not digest it well, dogs do not take change in food easy, you may expect some digestion issues, especially if the change was from one day to another, and not over a couple of weeks.
Please keep us posted and good luck!