Hi Jolu and a warm welcome to you from all the Staff and Moderators. It looks like ticks, possibly infested when in the nest. Hopefully they''ll drop off in a few days.... but there's so many I think he needs professional treatment. Most vets will give free emergency treatment to wildlife. We have some general guidelines
here for the care of injured and baby birds.
Please let us know how you get on.
I'm sure you will enjoy it here and I look forward to hearing your news.