The first thing a baby pigeon needs is warmth. Until they get their feathers, they're totally dependant on their parents to keep them warm, so you have to do that for them too. I don't know the exact temperature to keep them. I've done well at about 90 degree F. You can either use a small heating pad under them (not directly - they'll mess up a heating pad, so put you might want disposable or washable rags or towels over the pad, and then put the bird on that. Squabs lose a lot of body heat to the air so I'd place the pad/rags/bird combination in a box with a top on it. No matter what you chose, if you are using an electric heating pad make sure that whatever you use is not something that's going to burst into flames on you. Better a pigeon that never lives to grow up than a family lost. A thermometer placed inside the box should let you keep an eye on the temperature. If the pigeon looks "sweaty", it's too hot.
I have found that Kaytee Exact Baby Formula used in a very liquid state is a substitute for pigeon milk and is what i used as the baby get older I just make the mixture thicker