Lab becoming aggressive to other dogs

My 9 year old lab has started to become aggressive and growl/bark when other dogs walk past him. He only has to see another dog (usually his size or larger) and his hair goes up on the back of his neck. When i get about 100 yards away from other dogs he starts growling at them and then will start barking and want to get too them.

He always used to be a very submissive dog and never used to be like this, he was always the one to roll on his back and become submissive.

He is walked on a lead and it doesnt matter if the other dog is on a lead or not.

I am starting to get embarressed by it as the other dog is often minding his own business and Barney just wants to have a go at him

Barney has been ill in the last 6 months and has arthritis and hip and elbow dysplasia down one side so I know he is in pain.

Any ideas - or is it a case of him being old and grumpy (although he is 9, he is an old 9 if that makes sense)

My first port of call would be the vet for a checkup if he hasn't been recently, not only to check that he's on adequate pain relief but also that there's not something else going on - it's probably not helped by the pain from the dysplasia and arthritis but worth ruling out anything else contributing to it.
You could look into desensitizing him around other dogs like you would a fear reactive dog (this could be his issue, fear of pain)
http://www.spiritdog.com/reactivearticle.htm
(this link mentions TTouch but lots of the stuff they suggest is good to use even without TTouch)

Does it happen when Barney is off lead also?

If not, then it can just be frustration, he wants to get to the other dog and play etc. My dog does the same thing but it's simply because he wants to greet them.

If your dog is doing this off leash too, then it might be that he's in pain and is warning the other dog off. Is the vet giving him anything for the pain?

The key thing - and it's hard - is for you to try and stay calm when you see other dogs. The second YOU tense, that travels down the lead to your dog and puts him on the alert.

Have you tried diverting his attention with a 'watch me' command and then a treat...?

You say he has always been wary (fearful) of other dogs and is getting worse as his health gets worse. This is understandable. If he is afraid that other dogs will hurt him, it will get worse as his arthritis causes him more pain, won`t it?
I would first take him to the vet for pain relief, then give him supplements for joints like glucosamine and chondroitin, MSM and fish oil. You should also keep his weight down, and stop him getting overtired or chilled.
When walking, if he sees another dog and reacts, give him more room and detour so he isn`t too close to it. Then praise or reward when he stops growling.

thanks for your comments -

Barney is on Meloxidyl, Yumove and Aloe Vera, all to help his pain -

he is under the vets at the moment for his arthritus and will be starting another set of injections in the next week or so.

Too be honest, I get upset too see him like this as he is such a good natured dog and this has only started recently.

I think you could be right and it may be the pain thing, maybe he is warning other dogs to stay away as he doesnt want to get hurt anymore

I do try and keep as calm as i can and when Barney is off the lead, he couldnt care less.

He has 3 walks a day and 2 of the walks are along the seafront (he needs to be kept on the lead to stop him eating all the stuff that comes in from the sea and jumping in it!!), his third walk is to the park where he can have a little run (need to be careful how much due to his legs causing him pain) although yesterday he could hear a dog in one of the gardens and he went bounding up to the fence and started barking - but thats just a dog thing!

like the others have said, you probably already answered you own question,

dogs are just like humans, when in pain can be hostile.

I would also have his eyes checked out.

Yes Cherry has always been grumpy about dogs in "her space" but we didn't know she had entropion in both eyes as it was missed by our vet for a number of years