personally, in addition to what lifenotsoeasy has said.....
1. You are financially associated by the mortgage, so your credit rating/score could in effect drop to zero, that means you very likely wont be able to get any credit, that means things like a new gas/electric contract, mobile phone contract, sky contract, credit card, card loan etc etc - even monthly payment plans on things like car insurance - you might find you no longer qualify for them, or if you do at possibly silly high interest rates. [all that needs to be confirmed by one of the reputable IP firm reps that post here, or the mortgage guys]
This will have a marked affect on you financially over the coming years, likewise if you decided you want a mortgage of your own or to even rent a new property you may fail the credit check (may=probably) and wont be able to rent a new place etc etc. You wont be able to sell the property while he is in the IVA.
This is all worst case scenario, because you are financially associated, so hopefully it demonstrates the requirement to get some legal advice. I would suggest that if he wants you to sign the forms then he pays for your legal advice and then you go and get some from your own solicitor.
Come the end of the IVA if he hasn't repaid all the debt plus fees plus interest and he has more than £5k his share (highly likely in 5 years time of property market rises) in the property he must release that, this is normally on the paperwork via a remortgage, most in an IVA fail re-mortgage credit check (as mentioned above) so, if he is about to take out an IVA, depending on the company he is with the terms may say he is required to instead get a loan to release his share of equity in the house, once he has done this if they agree the IVA will be closed and then the house belongs to the both of you again to do what you want with it.
Of course he might not like the terms of the secured loan at the end and could say no, so instead that could breach the IVA agreement and then they come after the equity in the house by whatever means possible, if his IVA breached he could be made bankrupt and then they come after all his assets, half being the house.
It is a messy solution, and for you it will mean you cant move on with your life for 6+ years in terms of being associated financially, Personally I would not be interested in such a thing, I would be more inclined to say I'm sorry its not for me as it will mess my life up too much, I think we should sell the house, if you do that then you split the equity, disassociate yourself financially and then he can do what he wants going forward perhaps even use his share of the equity to pay off some of his debts, or you buy him out, you could probably be quite fair with him because if he disagrees and you don't sign the forms he is faced with bankruptcy which possibly sells the house anyway. If you can transfer mortgage into your name, you could then rent the house to him. While all this is going on he could use someone like step change to setup a debt management plan (DMP) which is where they write to creditors ask them to freeze interest, setup manageable payments each month for him, then once that is in place and the house sale done or transferred into your name then life goes forward and he can make financial decisions that do not impact you. The transfer into your name would have to be at a sensible price because one of the questions on an IVA application is have you disposed of any assets below market value in the last X years.
I am not qualified to give financial advice but hopefully the above give you an idea of possible hurdles and options.
But signing those forms in effect means you lose control of the property.
My Blog details, the route I took before IVA, how I choose my firm, equity release advice (year 4-5), challenging the CRA's keeping IVA on credit file once gone from insolvency register
IVA ended August 2015. Would recommend McCambridge Duffy