I'll echo that if he's heavily matted, taking to the vet or a groomer and getting him shaved down would be a better first step. Right now you need to bond with him, and combing out really bad matts can hurt! so let someone else take them blame. That way you can be the comforter, and when he then has enough fur to brush again, you can make grooming just part of your routine together.

Does he have a name yet?

And thank you for rescuing him from life on the streets! Right now you should take a bunch of "before" pictures, to constrast with how much healthier and handsomer he'll look a couple months from now!