Deed of Trust needed-HELP!!

I have been trying to get my head around this for almost a week now and just can't work it out.

I am adding my boyfriend onto my mortgage so we can raise additional funds for work on the house, clearing my debts and paying off the remainder of what is owed to my kids father.

I have owned this house for 11years which I bought with my kids father for £175,000 but only had a £79,000 mortgage.

8years ago I took over the house on my own and removed the kids father from the deeds. I increased the mortgage to £99,000 so I could pay some of what is owed to my kids father along with replacing my car and paying money back to a family member my and the kids father owed.

My mortgage now is £92,148 and the house has been valued at £220,000.

The only way I can raise the money to pay off my debts, my kids father and raise money for the work on the house is by adding my boyfriend to the mortgage.

My boyfriend is not able to add any of his money to the house so all the money needed will be added to the mortgage, so the mortgage will increase from £92,148 to £136,000.

I cannot afford the £320 on top of the property solicitors to have a deed of trust drawn up.

How can I work out what percentage of the property is mine and what percentage is the boyfriends?

Thank you