Ahh, how lovely!
I don't suppose you can see his nest? I was just thinking that Mum might be a bit wary, as you're still about (caring for the chick) I thought if it was close to your home, and not too high up, you might get a ladder and put him back in the nest? (Take care though, that beak can cause damage even at such a young age. I'd recommend wrapping him in a towel if you end up picking him up. Covering their heads if you have to handle them- making sure they can breath - tends to calm them)
I've rescued a few sparrow hawks here, and my local (well, it's about 3 hours away!) Irish Raptor Rescue Centre said that hawks/owls, can often damage chicks inadvertantly with their talons, when picking them up when they fall out of the nest.
Just a thought.
Well done for being such a love and looking after the little chickie! Good luck!
K