It would be good to help him understand that there is no rush. Jesus was 30 when he was baptised. If he says that this is different because we are so close to the end, that opens up a great opportunity to discuss the history of the religion's failed end time doctrine. Bring up 1914, 1925, the 1941 release of the book Children, and how the religion has always used fear that the end is near to rush children into baptism.
As his loving father and spiritual head (not by JW standards but by your spiritual standards) tell him that you are directing him for his own benefit to wait until he is an adult. Anyway, baptism is just the outward display. It is the dedication that really counts, so if he raises the fear that he will lose his live at Armageddon for not being baptised, remind him that Jehovah would know if he has really dedicated himself, and you can take the blame for preventing the baptism.
This could all lead to some really good conversations and help you understand what your son is really thinking.