a few days ago my staffy mated for the first time. since then he has been very somber. Is this normal?

Have you checked to make sure he is not injured, some could be described as arrogant but sombre would not usually be 'used' dog behaviour,. If you can’t inspect him yourself I would take him to a vet for a check over.

EDITED to say I assume that he was fit before the mating and has sound hips and that either you or the bitch owner are experienced in mating your breed of dog or you ask someone who is to attend. Because if the dog was not health checked before the mating them it may be that the effort was too much for him for some physical reason.

hello jackie,
on my part i am not an experienced breeder. this incident was an accident (see breeding)
thanx for your reply taz is now back to his usual self all somberness has gone. i think he was just all sh***ed out!