It is possible to include your son to your IIP application only if he is fully dependent on you. As your son has his own income, you can't add him to your application.
How can I include my 23-year-old son in my Malta’s IIP application?
I know that although unmarried children between 18 to 26 years old could be included in the application as a dependent, they need to be financially dependent on the parents. But recently I was told that if the child’s personal income is below a certain threshold, they could still be deemed as financially dependent and be included in the MIIP application. Is it true? How much is the threshold?