He can do that - but he is thereby wittingly or unwittingly making a trust which has complications. Depending on what he actually means he should seek legal advice on how to word it. It would certainly be better if he named a trustee and outlined the trustee's powers.
The danger in what he is doing if it's worded wrongly is that you mum will only have the interest on what he leaves to live on. She can't spend ANY of his money, sell the house for a smaller one, that sort of thing. She just has a life interest.
I'm not sure whether that sort of will can be challenged or not on grounds of unreasonable provision. My gut instinct says yes but I'm not that much of an expert
If he sets up a trust with your mother and the step daughter as beneficiaries and gives the trustee powers to balance the interests of the two then I'm pretty sure it couldn't be challenged.