There are no restrictions on flying with well-controlled diabetes
.However people with following conditions are not allowed to fly by aeroplane
....Uncomplicated myocardial infarction
....Complicated MI within 4-6 weeks.
.....Decompensated congestive cardiac failure.
.....Coronary artery bypass graft within 10 days.
....Cerebrovascular accident within 10 days
....Uncontrolled cardiac arrhythmia
If your brother doesn't have above mentioned problems,then there is no problem in his travelling by aeroplane,however it would be better if he talks to his treating doctor and take his permission.