Taking those 2 antidepressants together is a potentially dangerous combo -- definitely can increase heart rate and other sx that could interfere w/ sleep (and wt. gain). 10mg is also a hefty melatonin dose and could be counterproductive, causing headaches, for example, and/or shredding the natural sleep cycle.
I would back off everything, start over w/ minimum dose of one antidepressant, titrate to effect, only then considering something else.
As others have said, comfort @ night is key, too, simple things like height/angle of back, head, legs; are his heels fully protected, likewise his shoulders, elbows, hands. All of these could be suboptimally supported and causing discomfort. The right room temp, sheets, blanket. A low-voltage heating pad can help relax muscles. Our PALS cannot tolerate much weight on his chest and virtually none on his feet, so we use several travel pillows to support joints, fold blankets down, use only very light/soft material on and under him, etc. Sometimes you don't know what's wrong until you shake things up a bit.
Also, you did not mention breathing, if he is using a PAP, etc. If he does, there might be room to work with the mask/settings. If not, breathing could now need support. He could be waking up totally partially from air hunger/obstructions/spasms. Those start taking any kind of benadryl (the ingredient is the same in Tylenol PM, BTW), benzo like Ativan/Valium or non-benzo sedative for sleep (like Ambien) should read your data card carefully thereafter because it's likely your settings could need adjustment since each of these impairs breathing and affects sleep architecture.