Why does he need to be put on the lease?

It depends on if the landlord runs a background check, and they usually do.

Most apartments have rules against people living there without being on the lease. It could be grounds for getting evicted.

Read your lease.

I would think that as long as its a visitor staying for a period and that you let the landlord know ahead of time , that there would be no problems.

I had an apartment in my name only and later my ex joined me ( I did not add him to the lease).

When of course things escalated with his bad behavior resumed and his not working no paying anything it became clear it was time for him to leave.

He refused and behind my back thought that if he paid part of the rent that he would be considered on the lease, but not so.

My signed permission was needed and thus after he realized that I wasn’t going to add him

and much pains , he left after his newly formed friendship with another man, he got looked

after at his place , and it was that man’s problem then on.

Sometimes its a good thing to NOT add anyone to your lease as If I had the management

would not have any legal recourse to evict him.

You can avoid all of this if you consider what would happen is you have a falling out with your friend?

Avoid all this drama by just having him as a visitor for a time period like a visiting relative for

a few months .

I think there is a bigger concern. If he tries to get on the lease and the warrant pops up then I’m pretty sure the police will be notified of anything with his name on it. Now if you meant he has a minor criminal record then most of the time that won’t stop anything. Like if the criminal history is like speeding or a DUI. A person renting their home is never under obligation to rent to anyone. However, apartments in most states can’t discriminate against ex-cons unless the person is convicted of child rape or murder and can pose a possible risk to tenants or future tenants with children who can see who lives in the area. Child molesters are usually listed as public knowledge. Also, they can reject you if you have a history of skipping out on previous owners or has a really bad credit score.
The reason most businesses like to have all adults on the lease is to protect their property from damage and protect other tenants by always knowing who is living on their property and vetting them.
Your son could also be in violation of his lease. If another tenant reports him because his brother is making noise or getting into fights with neighbors they can both end up being evicted.
Also if it’s a building that has gas pro-rated by how many people live in an apartment then he has to report his brother as living there. Most places will only do a month for visitation max and after that they have to prove a reason. Disability, or someone caring for the tenant.