I don’t know about not building up a tolerance (not immunity). I’m horribly allergic to cats, but after almost years with the monster herd (a mamma who gave us 4 kittens that we kept) I’m doing better.
1) lots of vacuuming
2) limit where the dog goes, especially to the child’s bedroom. Have areas of he house where the child can go that are dog hair free.
Very limited exposure, with amounts building over time. They are now treating kids with peanut allergies by giving them peanuts every day, starting with extremely small amounts that gradually increase over time. (according to my allergy doctor.)
four years with the monster herd