If you are in the UK and you think your dog is crossed with a Pitbull I would keep quiet about it - at present they are still listed under the Dangerous Dogs Act as a 'type' dog (i.e. Pitbull type) and can be seized by the police on sight. I would ensure you have it neutered, insured and muzzled in public and make enquiries about getting him put on the register of excluded dogs. I would add I have no problem with such dogs but for your own peace of mind you would be doing all that is required under the law as he can be removed from you and destroyed if deemed to be 'of type' without ever having so much as licked anybody, such is the stupidity of Breed Specific Legislation. If you believe he is part Staffie then try to ensure you can prove that, as it would be a defence should he be seized. Maybe photos of his mum/dad if you can get them?