Maybe the pigeon is injured from flying into a wire, or similar.

He may have escaped a raptor attack?
I would not be too concerned with disease transmission. We cant live in a world where we are afraid of everything going wrong. If by some chance that pigeon is diseased, it already happened,
I really don't think he is. 
Feral wild pigeons are usually quite hardy, and the weak/sick usually die off.
Feed the pigeon scratch seeds if you have. Wild bird seeds are even better. Offer water to drink in a container about 2 inches deep. Pigeons drink differently than chickens. They submerge their whole beak into water You can also offer a shallow tub of water to possibly encourage pigeon to bathe. If he does bathe, it is a good sign of recovery. If pigeon recovers, from whatever may have caused him the (no flying) he may eventually fly away back to his feral flock.
Do you know where the feral pigeons in your area congregate??
WISHING YOU BEST,,,,,,,,,,,and

for taking care of this feathered friend.