Make sure your house has a crib too and 10 hearts with him or more. If you remove the crib for any reason, there won't be a prompt.
After that, every night there would be a one in 20 chance of him asking to have children or adopt (if same sex). After that, it will take 14 days until the baby is born.
But yeah, 3 seasons sound too long, usually they can start asking after one week.