@David.  You cannot make a particular field to be readonly for a particular user.  Odoo's security feature works with groups.  So, you need to first create a group and assign this particular user to that group.  Odoo itself does not provide a staight forward way to make a field to be readonly for a particular group.  You can make it visible to certain list of group by setting the groups attribute to that field.

To achieve what you need, you need to work around the available features.  I've answered similar question before: https://www.odoo.com/forum/help-1/question/how-to-inherit-a-view-with-the-purpose-of-hidding-a-page-from-a-group-73171#answer-73176 although in this case the questioner wanted to hide an element).  You can use readonly attrs to achive make the field readonly.