Title says it all really. They look really red and sore and I don't know if this has any relavance but the underside are really black too. It almost looks like their dirty. Any help would be appreciated

Has he been licking them excessively? Has he run through undergrowth, brambles, stinging nettles?

Does he live outside? (ie, sitting with them on a cold surface)

Has he had any change of health/diet recently? (that may irritate skin)

My older dog got very sore testicles with dry skin when he had major surgery, we had to put aloe vera gel on them