You say he has run up debts on the property. Are you sure they are secured on it - i.e. that charging orders have been obtained by the creditors? (I assume none of them result from documents you signed agreeing to the debt being secured.) If he has got debts secured without your signature and without charging orders, then I think he may have done something he shouldn't have done, such as getting your signature forged or some other fraud. If the secured debts are in both your names then payment to the creditors (unless it can be proved he has committed fraud) would come out of both your shares in the house. If debts in his sole name have been secured by charging order then they can only come out of his share - whether it is a joint tenancy or not.
If the house is a joint tenancy it becomes yours on his death; if the joint tenancy is severed it becomes a tenancy in common, the % owned by each party is specified in the documents & each can leave their part to whoever they like in their will. With joint tenancy you would normally be entitled to 50% of the equity so you should make sure that - if you do sign the form - you will remain entitled to at least 50%.
Whichever type of tenancy it is, I think (but am not certain) that all the secured debts have to be paid before the equity can be released. (The exception would be if the charging order debts exceede the value of his share.) If this is the case, it could be that the secured debts are less than 50% of the equity and that he is efffectively trying to do you out of something. For example house value �100K; debts �40K. With joint tenancy you would end up with �60K. With tenancy in common at 50% (assuming he leaves his share to someone else) you end up with �50K of the value less �20K of the debts - �30K.
I am not sure about the above, but you ought to get it checked out.