Any 1 got any advice on dogs staffys?

hi every 1, im hoping some one can help i not sure wot to do about my staff nasher of 11yrs , hes the bomb and has been a frendly and loyal part of my family all his life showed some aggrestion towards other dogs but never people having 3 children myself hes no stranger to the hands of little ones, hes been there dress up teddy on a few occations, lol !!!, 18 mths ago i got ruby she was 5 weeks old and he loved her they got on fab and still do only now in the past few mths he started showing aggrestion towards other kids and adults i no he has arthrites in all 4 joints and im always telling people to leave him be as when hes maybe lent on this is when he snaps hes also started growling at people when on walks ,he s not really hurt any 1 yet but they ve been really scared he would never hurt me that i trust but now i cant trust him with any 1 else im so scared he will hurt some 1 or 1 of my children if they lent on him and i dont no wot to do pls can some 1 help