How can I be a kind Alpha with a donkey?

My friend's donkey is, in general, very sweet.  I feed him regularly, brush him, and interact with him.  Unfortunately, of late, he has been challenging me.  He nudged me out of the gate and broke loose.  He chased the goats away while I was brushing him because they wanted brushing too.

I know pecking order is huge with donkeys.  How do I establish dominance without being mean.  

Do I not allow him to chase the goats away?  If so, how?

Thanks,
Heather