I'm thinking of raising the rent on a tenant with no notice, knowing the tenant is limited in income.
Do you think it is fair to raise the rent without giving notice?
No, but can I earn a lot of money from him.
You should always give your tenants appropriate notice when you are going to raise the rent, especially when they have limited income.
I think I have a good relationship with him and he understands me.
You shouldn't assume that your tenant will feel the same way. You already know that he has limited income, so he might not be able to handle the rent increase, and you might be left with an empty building. Furthermore, many states actually require advanced notice, so it is not worth the potential legal trouble.