I need advice on my poorly looking Collared Dove?

Hi everyone.  
I have alot of different birds in my garden and whilst working from home, my birds are my lifesavers and I feed, water and give them shelter as though they are my family. 

Through my binoculars I have been keeping an eye on a few of my birds and a collared dove is causing me concern. He walks around fluffed out with his head in his neck. Crouches down and blinks like he wants to go to sleep. He tries to eat but either picks it ui in his beak but doesnt manage to be able to get in far enough into his mouth and he then flicks it out ( I put down sunflower hearts but they seemed too big, so went and gto some smaller seeds. Have placed food in the areas I see him stop and put out water. 

When I come out into the garden, the other birds (collared doves. pidgeons, starlings ) fly off. He doesnt until I am about 3 feet away. But then as I turn around, hes flown back. He looks like a fledgling but his tail feathers look long so could be a young adult.  Tonight at dusk he was joined by another slightly fluffed up dove, who was eating and stood more up,right,  they didnt walk around together but again when I walked out they flew off in different directions.

Over the weekend, I was monitoring a fledgling/young adult starling. He was crouched aswell  but found it difficult to fly, each night he went somewhere but Sunday I didnt see him fly off and couldnt find him. On Monday he was under my Jasmine tree and he had died, I blame myself, could I have done something and I have cried alot about the beautiful bird.. 

 I have been supplying mealworms and crumbled suet for my starlings and feed their kids now so I can recognise when they are learning to eat.. open beaks at ground and not picking anything up. He was like this and only managed to pick up small amounts. 

Sorry for this to be war and peace but if there is something i can do for the collared doves, I would love to hear from you.

Thanks

Alison