The original post should have been clearer for clear advice.
As a Muslim - one must follow the rules of inheritance so Google Muslim inheritance calculator and see how much percentage would go to your husband and how much do your son if you passed away.
In terms of UK law, your husband wouldn't be entitled to anything as you aren't legally married in the UK (you can just go to the registry office and show your nikah certificate to make it recognised in the UK).
I wouldn't put any money into your husband's house right now. Think it through first. If you are not on the deeds, you are entitled to 0% if you split up. Insha Allah it won't come to this. But if you are putting money into the property, get your name on the deeds or whenever you need to do. Work out the percentage eg he owns this percent and you own this much because of how much you've put in for the extension.
If you are paying for the mortgage regularly then add that to the percentage too.
Islam recognises individuals having their own things so eg just because you get married it doesn't mean you get 50% of the house but according uk law it goes.
Both of you need to be just and fair and work out what to do.
Seeing family go through something recently, I would say either buy a property in your son's name or put it in a trust but tell him that he is the owner of it as Islam requires this otherwise it goes into your estate and must be split between husband, son etc.
Many Muslims don't really follow Islam properly but when it comes to inheritance they suddenly want to follow Islam when it comes to this ruling as it means they get a lot of money.
You really should consult a scholar who knows about this.
If you want to protect your son, maybe buy a house in your name and his name but tell him he's the owner of it 100%.