they're alphas who cannot physically control their instincts to pounce on any omega since that's their nature, so those little gesture can be quiet dangerous because of their different genders.
oooo see I see but don't they find out gender at a later age? pretty sure this was when they were younger so how do parents know they are alpha?
I'm guessing they were being strict because they really wanted to get through to him that he shouldn't be close to his alpha brothers who could lose control from his pheromones. It'd be really bad if one of his brothers accidentally bonded with him.
I just have one question, if his parents wanted to protect him why would they yell at him for holding hands with his brother?