How long have you been together? That would influence my decision if I were you. Did you get together just before getting pregnant, so perhaps you have only known him for under two years? Or was his divorce many years ago and you have been with him 5+ years?
You have paid £50k and have £100k left on the mortgage, but I wouldn't go just on the mortgage: I suggest you actually get the house valued by a couple of estate agents.
Say it is now worth £200k for example, and you bought it for £150 with no or minimal deposit, you now actually own half the equity (£100 owned, £100 on mortgage), because the increase in value is yours.
A deed of trust relates to the house ownership, not the mortgage - this is important - it says what % of the house you each own as "tenants in common", or that you own the first £X fixed amount and the rest is split in a % each.
This is regardless of who is on the mortgage or who pays the mortgage - although the mortgage company will have some input into these decisions.
(Get this checked by a specialist solicitor as I am not a lawyer.)
In the scenario above, you might get a deed to say that you own 75%, which is equivalent to the £100 you already have and half of the other £100, and he owns 25% which is half of the £100 still on the mortgage.
Then his name goes on the mortgage (likely you will need to get a new mortgage) as an equal partner and you pay the mortgage out of a joint current account.
(How you each pay into the current account and how much income you each keep for yourselves for personal spending is a whole other conversation).
This is not watertight, as in the case of a future divorce, especially after many years, everything is on the table and the courts will decide, but the deed of trust may well be given weight as it shows your clear intentions at the time of marriage. You may find that you would have to sell up to pay him his share.
Finally, your % of the home as described in the deed is what you leave in your will - another trip needed after marriage to a solicitor who specialises in wills.