Roommates

@Farah_sh_05 You'll need to make him homeless and not part of a household with other Sims before he'll agree to be a roommate. You can go into Manage Households and split him from his current household and then either make him homeless from there or temporarily place him in a new lot and then evict him. As soon as he is homeless and not part of a household with other Sims you should be able to invite him over and ask him to be your roommate.

First find the Sim in the Manage Households menu. I'll use Paolo Rocca in the Partihaus Household for this example. Use the Transfer Sim between households button that looks like a double arrow to get to the Household Split and Merge Screen

Select the Sim you want to split from his current household (I'll click on Paolo Rocca for this example). Click on Create a New Household and use the arrow keys to move the Sim into the new household. Give him a few Simoleons in the Transfer Funds option. Click the check-mark.

When you are done he will be homeless (Currently Not in World) and not part of a household with other Sims.

Now you can ask him to be a roommate :D

EDIT: One other important thing! After you split the Sim into a new household make sure you keep him as a Favorite in the My Households tab in the Manage Household screen and not in the Other Households tab. As a roommate he will still show up as being in his own household (not your playable household) and keeping him in your My Households tab will keep the game from culling him if you get too many Sims in your game.

Thank you! 

@Farah_sh_05 Let me know if this works for you & if not we can try something else.