From what you have said, I would say it is most likely down to fear aggression and that he previously got confidence from the other dog.

I would suggest seeking the services of a good behaviourist, who if they mention pack theory and dominance, run a mile, and in the mean time, ensure he is under proper control and probably muzzled when you take him out, just in case.

A good book to help you understand dog on dog aggression, an dfear aggression is 'Fight' by jean Donaldson.

Well done for recognising a problem, and dealing with it, and sorry for the loss of your other dog.