You cant return a chick to the nest after handling it for so long. Just check online for the species and feeding habits. The warming blanket is a good idea but keep it fairly low. Chicks can dehydrate quickly. They need very very frequent feeds and can be overfed easily. best go online, if youve seen the mother, common pigeon, wood pigeon look up specific breed. Good luck.
Find a way to put him back in his nest maybe