First of all, keep in mind that I am unable to diagnose you because I am unable to examine you, this forum is for educational purposes.
The symptoms that you describe are non-specific, and cover several different trains of thought. First your headaches have characterisitcs of either low pressure CSF headaches (as from a CSF leak either spontaneous or caused by a lumbar puncture) or migraines. A low CSF pressure headache is relatively mild with lying down and then becomes painful with sitting up and even worse with standing. These headaches are very common after a lumbar puncture and can be treated with fluids and caffiene. Less commonly there can be spontaneous leaks around a nerve root, at the nose, ear etc, which results in chronic CSF loss. This condition can be difficult to diagnose, but CT myelogram is probably the most effective diagnostic tool. A blood patch and ACTH are often used to treat this condition. A second possibility is that yhour headaches are migraines. Migraines can have a positional component and is related to increased central sensitization. To investigate the migraine angle I would start a migraine prophylaxis medication such as elavil, topamax, verapamil, nadolol, etc taken daily (begins to work in 3-4 weeks). It is unclear the extent that your bone tumor might be playing a role in this. If any role at all, it is likely triggering your headaches by pressing on a branch of the trigeminal nerve. As far as your nausea, low grade fever and abdominal fullness/tenderness, I would suggest that you see your PCP (primary care provider) who has more experience in this part of the body than a neurologist. There are neurologic diseases that can affect the abdomen as well such as sarcoid, neuroblastoma, lymphoma, but I do not suspect these from what you have described.
I hope this has been helpful.
Sorry to hear about these problems. At first glance, it is hard to know whether the two complaints are related -- it seems rather unlikely. But regardless, abdominal fullness with change in bowel movements is worrisome. It may be as benign as bloating or as concerning as a growing mass, irritable bowel syndrome or peptic ulcer disease. These situations are tricky when one has no insurance. But the best idea would be to take him to the ED -- most hospitals will have to give emergency insurance coverage to patients who are admitted to the hospital. The problem may be that some hospitals may not want to admit a non-emergent patient but that would then depend on how you talk to the ED docs down there. The alternative would be to find a gastroenterologist who would be willing to perform the most focused and cost-effective workup without, obviously, missing anything.
I wouldn't worry about the connection, I would be more concerned about the abdominal symptoms themselves.