It seems only Allah knows, for thats what is being discussed all over the Muslim world and even amongst some Christians. But I, in particular, suggest that we should wait to see how he will be buried. If he died as a Muslim he will then be buried according to Muslim rites; and if as Christian he'll be... Right?
I hope he died as Muslim.
He may not be necessarily buried according to Islamic rites Muhsin, even if he is a muslim now. The important thing is that as you said, only Allah Knows and He is the best Judge. Whether Islamic or non islamic burial rites, since he (michael) cannot dictate how he should be buried, then we cant judge him based on that.