Is a father obligated to send money to his adult son (18 years old) who is capable of supporting himself, especially given that he only began spending on him two years ago and is divorced from his mother?

As long as your son has reached puberty and is capable of working and earning, his financial support is not obligatory upon you. However, assisting him with money to meet his needs or for trade is a good deed. It was a dereliction on your part not to spend on him before he reached the age of sixteen if he was poor, and you are obliged to repay what was spent on him during that phase if the one who spent requests it, unless the one who spent donated the expenditure.