Effectively he is dividing his assets in half - with his half going to his children from his first marriage, and the other half going to you and your children.
If you jointly owned the house, he could leave his half to his 3 children but then you would have the remaining half. If you chose to leave this to your 2 children - so your joint child and sole child - then there is a likelihood of unfair distribution: his children would get 1/6 each, your joint child would get 1/4+1/6 and your own child would get 1/4 [assuming neither of you need social care and the entirety of the house is available to leave at all].
Theoretically I assume this was meant to be an attempt to trying to treat your child the same as his in the will (ie ensure every child gets 25%). This assumes that you will leave that share in its entirety to your sole child, when in fact you will likely split it between both your children. The result is likely to be that a) your sole child, his step daughter, will not benefit until you die, b) they will actually only get 12.5% of the total pot [half of the 25% you inherit], c) your shared child will ultimately inherit twice - 25% when DH dies and and further 12.5% when you die - and d) only if you have not had to spend the money on care bills before then.
This set up will likely cause conflict between your shared child and ALL their siblings, I’m afraid.
Additionally, he is making sure that you have to stay with him for the rest of your life in order to benefit from the asset, as you would unlikely be awarded half of the marital home in a divorce at this stage in your life. It also means that when he does die (if he does not outlive you) you will only have 25% of the value of the property to invest in a home of your own, unless he has also given you a lifetime interest (ie right) to remain in it until your death. This may not be an issue if that 25% is enough to buy a retirement flat for one person.
However, if he does outlive you, your sole child may ultimately be excluded entirely as he can rewrite his will to divide it between his own three children.
Then, if you have a right to remain in the property after his death, whilst only owning a 25% share, then he needs to be clear upon how maintenance and ongoing costs of the property would be split - because he would be imposing upon his three children the financial and legal responsibility to maintain the property during your continued occupancy (which I think creates a host of legal issues and may create conflict should any of the three be unable to afford to do this).
Not sure there is a perfect solution as this is the sort of complex issue that arises in a blended family scenario, and is complicated by you not being named on the title deeds. It is not unreasonable that he would want to protect his assets for his own biological children but I think you might want to speak to a legal advisor and ask what your rights are in terms of where you live if/when you outlive him?