Can your boyfriend transfer a yeast infection back to you?
Yes. If you can transfer it to him, he can do the same to
you.
Candida yeast is present in the mouth anyway; he does not even
need to get it from you to develop it on his own.
It is often already present in the vagina and mouth of females;
it can become an infection for many reasons, one of which is poor
hygiene practices by either person, and can be transferred
constantly between them.