Ex paying direct payments, renting out property, advice?

Hi my ex is unable to work due to a criminal conviction he has. I'm assuming he's on incapacity though he's never told me what he's on just that he cannot work due to the conviction. We went through direct payments with the calculations made by csa. I was aware he was no longer living at his property due to neighbours knowing about his conviction, and was living with. Family member. I assumed he had sold his house but I've recently been told by him that he couldn't afford to get our son a replacement Xmas pressent due to not having money. When I asked him how as I asusned he had money from the sell, he explained that he rents out the property. On our direct payments it mentions no other income just benefits. Would it be on the forms if he had?

I'm not sure how to progress. At the minute he rounded up his amount he pays monthly to £30 so he pays £2 more than the calculater suggested. Is what he's done legal? Would it affect the calculations? His house when I was with him was calculated to be worth about 90,000 and is a 3 bedroom terraced. I don't want him not giving the proper amount -our son is disabled, and tbh his father literally does nothing else for him. He's only allowed supervised access an hour a fortnight. But obviously if I ended up going fully through csa I would be down £2. My ex was controlling and emotionally manipulative so I don't want to rock the boat if what he's doing is totally above board /won't affect anything.