Is an employee required to return company assets (internet, laptop, and printer) after leaving work, especially if the company has withheld his due bonuses?

First, you must verify the financial right of your brother over the company. If it is proven, then it is permissible for him to keep the company's properties in his possession, provided that their value does not exceed his proven right. This is known as the issue of al-Zafar (taking one's right by force when it is withheld). However, if the amount of the bonuses was already established as due to your brother from them before his military service, then there is no doubt about the establishment of his right.