Pass. I don't know what he has agreed as part of the IVA. I presume that if at any time the IVA is not kept to, that the Insolvency Practitioner could take forward actions such as bankruptcy against him. From what I have read about IVA's, some assets are ring fenced for protection, so this might include the house in joint names.
I think you need to see the IVA agreement that he has and then to make further enquiries with Citizens Advice or National Debtline.
The following link concerns charging orders, but it may be worth reading, as it does cover joint property and debts.
http://www.bankruptcy-insolvency.co.uk/debt_factsheets/charging_orders_in_the_county_court.php