There are a few special rules for who is considered to be a dependent for medical expense deduction.  But your post does not contain any of the information that we would generally use to evaluate who can be claimed as a dependent

In particular, you don't mention the sons age,  whether the mother paid for over half of the sons support, etc.

The requirements to be a dependent of medical expenses are laid out in IRC 213(a), which references IRC 152, with specific provisions removed, however most of the usual provisions remain. The provisions which are removed are commonly referred to as, The dependent of another tax payer,  the joint return test, and the gross income test.

My speculation is that the son would not qualify as a qualifying child due to age, and would not qualify as a relative due total support, but those are of course purely guesses, because you didn't state their age, their income, or the amount of support provided.