Mark - we don't deal with wills or % shares so on that score I would recommend that you seek legal/financial advice as appropriate. Varying the will and % shares does impact on your beneficial ownership but does not impact on the legal ownership, that we register, unless you then make an application to update the register.
If I have understood correctly the property is/was registered in the joint names of your late husband and yourself. If you want to now transfer that legal ownership to the joint names of yourself and your son then you will need to transfer the whole title from yourself as the transferor to yourself + your son as transferees -
Registering land or property with HM Land Registry: Change the registered owner name - GOV.UK
If the register still refers to your late husband as a joint legal owner then you will need to include a certified/official copy of his death certificate with your application
A further point to consider is whether there is a form A/joint ownership restriction on the register to protect the 50/50 split of the beneficial ownership/your wills/any tenancy in common created. If there is then this will remain after the transfer I mentioned.
That doesn't define you and your son tenants in common but it does stay on the register and will protect any % split you and your son may agree to and formalise in your own wills for example
I assume from the Q that there is a form A restriction on the register - if that's to be cancelled then our PG 6 section 8 explains how to apply to cancel same. The form RX3 and ST5 can be submitted with the application to register the transfer.
Practice guide 6: devolution on the death of a registered proprietor - GOV.UK
Transferring the legal ownership is fairly simple but it's important that you both understand the split/wills/trust that exists, protected and you both are aware of it's impact and what can happen in the future re the property. Hence the recommendation to seek legal/financial advice as appropriate
