I would let him approach you instead of you going after him. It might mean taking a lawn chair and a good book out into the pasture and just sitting there, maybe with a pan of some goodies at your feet. Donkeys don't respond well to the typical flight-based horse training methods. They are a different species and they evolved with different behavior patterns.
I know you bought him as a companion for your colt, but until he's gentled down I would keep them separated. You want the donkey to look to you for companionship and not the colt. That can come later, and it won't kill your colt to wait a little bit. If you can put him in a round pen and just sit with him for an hour or so each afternoon, eventually he'll get curious and start inspecting you. They love butt scratches, and also if you can fold your hand up into a "duck mouth' shape and scratch the insides of their ears- you'll have a best friend for life
I really recommend Sue Weaver's The Donkey Companion
for first-time donkey owners. It's chock-full of information.