German shepherd, need help asap please

Question
Hello, well we have a 2 year old  male German Shepherd.We have had him about 3 months and have found out that we are his 4th owners, we think its because he is very big and other people coulnt cope with his size.anyway the problem is that when i walk him and he sees another dog he goes mad,he growls,barks, pulls, stands on his back legs and it seems 2 be getting worse.i dont know what 2 do?I have tried 2 get him 2 sit when he sees another dog, which he does but then will go mad as soon as the dog gets closer.I have tried walking the other way when i see another dog coming but he just stands there staring and wont listen 2 me or walk forward.
Inside the house he is brillant, he sits, lie down, gives his paw when asked.Also if he is in the garden and another dog goes past he dosent do anything, not even bark.
Im just worried now as i dont know how 2 stop him going mad, i get dirty looks when it happens and its getting 2 the point where i dread walking him.He is also like this this with my husband when he walks him.
He has a holty lead which stops him pulling, also he hasnt been neutured yet, is this the problem or is there more too it, was he goes mad?
Please advise as we are at our wits end.
Kind regards and thank you

Answer
Getting him neutered will reduce dog aggression,so do that first

Then what you need to do is to get him a 'prong' collar,it looks awful,but it's not,it actually reminds the dog of being scruffed by it's mother-anyways when you get one of those and he acts up pull him back and say 'leave it' loud and firm

Another thing that also works for dog aggressive dogs is to go to someone's house that you that has a dog,and let him go in the yard of the dog,with both dogs wearing muzzles,so they can't hurt each other-but do not have a dog come to your house,as it will make him territorial

One more thing that might help is dog training classes-as a trainer can often help by seeing the problems you are having with him-some dogs will never like other dogs,but will at least not be aggressive,and by taking these steps you can make life eaiser for both you and the dog