Worried about my dog please help...

Hello I'm contacting you because I seen this place a couple of times and my dog been lately worrying me we havent been able to take him to the vet reason because we don't have the money so I wanted to know how bad is this...My dog belly is red and he has some black spots as well. I'm afraid that it might be real bad he always scratches and sometimes I see him scratching his butt against the ground but only for a few seconds. We given him a bath but it doesn't go away...