L.R.
I agree with others that there is confusion here -- why can't you know where they are going? Nothing can stop your son from calling you and telling you, can it? If dad stops him from contacting you at all when with dad, you need to get to your attorney about that! If you knew where they were, what would you do -- go get him? What is the custody arrangement and is it in writing with a court? If your ex is physically endangering your son you should be calling your attorney already.
I know what scoliosis is, and a kid who is going to have surgery for it surely should not be sleeping in a tent in a sleeping bag on the ground, or on some saggy old cot in a cabin, should he? Good grief, it's a spinal problem and he's weeks from surgery for it! Have they actually left on this camping trip yet? Is it possible to get the doctors to call your ex and tell him that your son could be injured or surgery delayed if they go on this trip? Or would ex just ignore it? (I wager he would.) Are things like rafting or other activities that could injure the spine involved here? Too many questions.
Dad apparently has a lot of control issues, if he is always wanting to pull your son off meds that your SON, not just you, see as beneficial. Has your son ever told dad how the meds help him or is son too cowed by dad to discuss it? Does son stop his meds when with dad? (Very, very bad idea, if that happens, and a reason to have the court intervene, possibly. I would think that "My ex tells son to stop meds when with him" would be a huge red flag to a court.) Your ex is putting your son in a horrible position of being torn between what son knows is healthy for him and what dad insists he do.
So: Has dad ever been involved in medical decisions? Has he ever been asked by you to attend meetings with doctors about the ADD etc, and about the scoliosis, so he understands your son's conditions? If he knew the seriousness of the scoliosis, what it is and why surgery is needed, could it change his mind even if he's grudging about it? It is too late now to involve dad like that but it might help next time.
Your son is 14, and in most states that is the age where kids get the right to be in court and tell a judge what they want in terms of custody. It's time to get him involved in that. Maybe a judge needs to hear from your son that dad is an issue when it comes to meds. It won't necessarily make a judge change custody, but it could get some words from the judge that could shake up your ex.
Please clarify for us what the custody is like and what's going on with the trip.