Hello and welcome to the site. Thank you for your question.There should be no tax implications of having a legal charge over your son's property. I presume it is to protect some sort of funds you would be advancing to him. If you are charging him any interest on that loan then the interest you receive would be chargeable to income tax. If the money advanced is free of interest, then there are no tax implications at all.
I hope this is helpful and answers your question.
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