Thank you for your question
How much is the house and why will your son not agree to the sale?
Your starting point is to get a valuation of the property.
You then have to write a formal letter to your son asking him to agree to a sale of the property.
If he refuses or does not respond after three weeks or so then you can apply to the court using a form N208 available here
and pay the fee of £355
I hope that this is of assistance - please ask if you need further details