My case is similar in that there are not DMPs within 1000 km of my partner. I contacted the VISA OFFICE in question and they said that the form can be submitted without the medicals and they will send information for him to take to his own doctor to complete the medicals. Meanwhile, our situation has changed, and my partner will probably get the medicals on a visit here (to Canada) instead... but we will likely still use this option for his children.
Gerry, you might want to contact the Buffalo office directly, and get something in writting (i.e. email) as to this option to exclude the medical in the original submission. Also, I presume that getting the medicals done after submitting will likely extend the processing time of the application, so you may want to reconsider if there is any option of getting the medical earlier via a trip to a city with a DMP.
From what I understand, the reason your girlfriend's son needs the medical is for in-case you ever decide to sponsor him in the future. If he doesn't want to keep that option open, I think you can include some kind of letter acknowledging that you abondon your ability to sponsor him and the medical will not be necessary... (this is where i hope some of the experts / seniors chime in...)