It doesn't matter if there is no agreement or if the money is made under the table, it would still be considered gross income. It should be included in the child support calculation.
There is no question that rental income CAN be considered income as long as it is a regular payment. If in fact he is contributing towards utilities or other costs, you can try and argue that the parent's costs have been reduced as a result and therefore, his income increased, so to speak. Keep in mind judges have a lot of discretion on such issues but it's always worth making those arguments.