My other thought is that some medications have short term memory problems as a side effect. Is your brother on any medications ? If her is, he could go to www.drugs.com , type in the name of the drug and see if memory issues are a side effect. Finally, is your brother having sleeping problems ?
For some vehicles, you're advised to change the coolant every 30,000 miles. For others, changing the coolant isn't even on the maintenance schedule. For example, Hyundai says the coolant (what many refer to as antifreeze) in most of its models should be replaced after the first 60,000 miles, then every 30,000 miles after that. The interval is every 30,000 miles on some Mercedes-Benz models, but on others it's 120,000 miles or 12 years.
More important than the type of antifreeze you use is to maintain the cooling system properly by maintaining freeze point protection and proper coolant level. Almost all coolants work best at the ideal freeze point mixture, which for most parts of the country means a 50/50 antifreeze-to-water mixture.