Very poor. It's actually quite incredible that he's survived as long as he has. The cause of death in patients like this is often respiratory failure as a result of infection or aspiration pneumonia, or status epilepticus (a constant seizure that cannot be stopped or controlled by medication) which causes cardiac arrest. Also, sometimes patients with epilepsy, even if their seizures are well controlled, just die without apparent cause. It's a phenomenon called Sudden Unexpected Death in Epilepsy Patients (SUDEP), and it actually accounts for 10-20% of all epilepsy-associated deaths. It usually happens during sleep; the person is feeling fine, just goes to bed as usual but never wakes up. At autopsy, there are no indications of a seizure immediately preceding death, and other findings are noncontributory: microscopic abnormalities in brain cells (which is a common finding in all epilepsy patients, not just those who die from SUDEP), pulmonary edema (fluid in the lungs), and mild but non-fatal changes in the cells of the heart are the most frequent. It happens, and it happens to people in way better shape than this poor kid. If his ultimate cause of death is epilepsy-related, though, I don't think it could be properly called SUDEP. There's so much other stuff going on with his brain alone that it's not accurate to call it "unexpected". There's a thing called SUDEP+ which accounts for epilepsy patients who do not die of a seizure but have another condition that could have contributed.
Matthew Nel, who died recently, was another child with a similar condition who lived much longer than doctors predicted, despite his best efforts. Mind you, this isn't really a good thing. The quality of life of children affected by such devastating neurological disorders is basically nil. At some point, all of the drastic interventions become something that caregivers are doing to a child, not for them. Matthew tried to die on a near daily basis, and was resuscitated by his parents every time.
Exactly. If they're high functioning enough (which is not at all a given), people with DS are at least aware of the dementia connection. Most will have even watched friends or acquaintances with DS slip away as a result of Alzheimer disease.
Alzheimer disease isn't the only condition associated with Trisomy 21, either. Children with DS have higher risk of congenital heart defects, including types that are unfixable/will kill them, and unfortunately, it can be very difficult to get an organ transplant for someone with an intellectual disability. A particular subtype of pediatric leukemia (cancer of the blood cells), called acute myeloid leukemia, is 150 times more common in children with DS than in unaffected children. Interestingly, though, their prognosis appears significantly better.
It's not just the intellectual disability. I don't blame parents who don't want to possibly have to grapple with putting a kid through treatment for cancer or a severe heart defect. Serious illness like that are traumatic and life-changing for children who are intellectually normal.
