i have a 5 month oes who has the habbit of when we are out jumping at people and mouthing them ,he is teething at moment forgot to say why doing this hes doing a excited bark ,any advice gratefuly recieved

Firstly I would keep him on a short lead so that he can't jump up or mouth anyone, otherwise this becomes his routine and presumably he is going to be quite a large dog.   If he is treat oriented then make him sit as someone comes up to you and only when he is good and quiet you give him the treat. Do that often enough and it will become the routine for you.  Another good thing to do is get the person coming up to you to give the treat as he is sitting - difficult to do I know when you don't know the person. I am sure there will be other good suggestions for you soon.