Your brother is not a dependent child because he is not a full time student without a break since before the age of 22. If he had done his army duty before age 22 and after that had been a full time student without a break, he could have been considered dependent.
You can still try to include him in the application based on him not having a choice in the matter and still being dependent but they may refuse him.
If you live in SK or MB, you can sponsor an adult sibling under PNP family stream. You would have to have lived there for a year and you and your brother would have to meet certain other requirements.
So if I apply for the sponsorship after he finished army (in half a year) and will be accepted in the University (for a post graduated diploma- mom will pay for his full time education) ?