circling other dogs

hi again, having a problem with jack. were to start. we recued jack from shelter 18 months ago, he is three years old and neutred. he is a friendly adorable staffy cross, who has always enjoyed a good run in the park, being friendly with all dogs. although when playing with meg our other dog (mongrel) they do snarl and growl when they are runnig around with each other. ( i dont like it, but have been told they are only playing. i dont like it if he does it with other dogs as some owners get worried - he is after all a staffy !!!!). anyway i digress. lately he has been very possessive with his ball, but he will also go and steal other dogs balls...although he has his own in his mouth. he has now startted running upto dogs, circling them and then sometimes barrelling into them and knocking them over. He recently knocked over a old spanial and bit her ear last week. He has been on his lead since, as i don  t trust him now. here is the thing...i have just been told when he was staying with my stepson last sept he was out in the country and he killed a sheep, im devasteted...... after going ape s... and a very strong lecture to my son, i now have to pick up the pieces and sort our jack out, coz he has captured my heart and i wont to do whats best for him. any advice for me would be most welcome.
ps sorry for the long story.   

You could invest in a training line, that way he can still have a run around but have control. Lots of dogs are possessive over toys but I'm not sure about the sheep   

          hi ive tried the long line, but he runs off that quickly he gets tangled up in it every time, very frustrating. but thanks anyway .

I will only use the long line if Katy is in her harness otherwise it gets wrapped around her legs   

Dogs can be very vocal in play, especially staffies so I'd try not to worry or be nervous about the noises that they make in play, as long as they are definitely just play noises

He shouldn't be allowed to steal other dogs toys, that's just asking for trouble. This is why I don't like dog parks lol, that many stranger dogs and resources in one area concerns me! You need to learn how to use the long line, you can't be allowing him to bully other dogs but he obviously still needs to interact and have his space that he's been used to.

Him killing the sheep will have no effect on his behaviour with other dogs, the livestock issue is a completely different issue. Does he generally have a high prey drive? Why was he off lead around sheep anyway, and why has it taken this long for him to tell you about this incident?

have you considered using an extender lead ? We have one for Rocky and he can now have a run about while reamining on a lead and under control. I would recommend a tape one rather than the thin line/wire ones you can get, the tape ones are more easy to see ahead:

http://www.amazon.co.uk/Flexi-Compact-Comfort-Lead-Black/dp/B001BYJ0MS/ref=sr_1_10?s=pet-supplies&ie=UTF8&qid=1394023297&sr=1-10&keywords=Flexi++lead

As far as I'm concerned, it is a complete certainty that my dog (as with many many dogs) will chase a sheep if he has the opportunity. It's not his fault - to him it's just innocent thrill. I very much doubt that he could be trained out of it. Therefore, he absolutely MUST be on-lead around sheep - and a short lead at that. If the farmer has concerns about a dog off-lead around livestock, he'll shoot the dog anyway. By protecting the farmer's sheep, I'm protecting my dog. That's how I look on it. A dog is a dog is a dog. Owners should know better.

i quite agree with you over the sheep issue. i was so angry at my son for letting jack off the lead  whilst out with any livestock.apparently ALL the family knew about the incident and kept it from me, as they knew i would be so upset. needless to say i have explained to my son about the rules of the countryside and livestock, i explained that the farmer wouldve been well within his rights to shooot jack.
i have him booked into training classes and am keeping him on the lead around other dogs for now. i have to learn to trust him again.
thanks for all the comments
ps i have just bought him a harness and will try the line on him with the harness.
I have him booked into some