And agree with Ciutadella you have no idea how things will alter etc.
One thing we did when we made a will, I think it was when dd was a preshooler (before we adopted) was to designate who would care for dd if we both died. Very important to do. At the time we both had parents and sisters and needed to choose who would be best for dd. In the unhappy event we both died she would have inherited our home and life insurance and my sister, designated to care for her, would have needed access to this to bring her up.
We added a clause that the amount would be split equally with any future children we both had.
IMHO this is a very important thing to do even though the likelihood is exceptionally rare!
Solicitors will be well used to dealing with all the complexities of family life so my advice is to make sure you use a good reputable one and do not attempt any kind of DIY will.
Your husband should really not be telling you what to do with your will or your share of a jointly owned home. It is totally wrong.
If you choose to share equally with all kids, placing your son at a disadvantage potentially, that is your choice. And your step children may become so dear to you that you choose to include them in your share.
But his insisting is plain wrong in this circumstance.
If the kids all lived with you jointly, if they don't inherit from their mum, these factors could all change my views but based on what you have told us, Rhiannon, your husband is being very presumptuous to attempt to make your will for you.