And yet, of course, he knew no other life, as his disease was from birth, so to him it would have been 'normal' in a sense. Whether he knew or thought his life would be short or he'd never be tsar is speculation though. In her letters the Empress would often talk about how they had to be strong for Alexei so that when he is Tsar he won't have to rectify all their mistakes, that they must hand over a strong country to him etc. Of course she may have been deluding herself, not wishing to think that, 'Well, our son will probably die long before we ourselves do and will never reign.'