I don't think it is too young, but it would be reasonable to expect that he wont understand what is going on when the time comes because of his age, especially if he is a sensitive child who may get upset at seeing you in what they think is pain kwim? I think it would be beneficial to do some role playing, simulating the noises you could make during labour and even go so far as to have his support person there to help explain what is going on to him, just as they would if it were the real thing. And also show that at the end of all the 'noise' that there is a baby born, using a doll, so he can understand that the noises help you get the baby out. So when the labour starts, he knows that it is normal and that mum is OK. Have a read of this article too https://www.bellybelly.com.au/birth/siblings-at-birth