Why does my horse have 4 swollen legs?

I put my horse in a new paddock with a horse that he is very wary of as he is above him in pecking order (trying to make him not so dependant on his current paddock mate.) After a few days I noticed that he seemed very lethargic. He was eating his hay with his eyes half closed. He got a nasty bite from the other horse, something that I found strange as he was usually very fast to get out of the way. Finally his legs swelled up. The back ones were worse. In fact the front ones weren't really showing any sign of swelling at first. He is nearly 9 yrs old. The paddocks that he lives in are about 4 acres in size. I see him moving throughout the paddocks and grazing as normal. He is still alert should something alarm him. I put him back in with his paddock buddy. He does not look as tired now however his legs have swollen up worse. One front leg is definitely swollen and the other I think is just a little as there is heat behind the fetlock joint. There is heat from the swelling. This is the 3rd day they have been swollen. He does drink a lot of water and go to the toilet frequently (he has for many years now). He did lose some weight this winter (I thought that it may be because he is more active due to stressing whenever I take his paddock buddy out). I am currently trying to get more weight on him. He did get the swollen leg problem some years back when in with his paddock buddy (no change of paddock) and no other symptoms (I had a vet out who had said that unless we start doing a lot of expensive tests it will be difficult to know what the cause of the swelling is). Prior to putting him out with the other horse I had noticed that he seemed to be a lot more chilled out around the paddock...not always running around when the other horse did. I thought that he was just finally maturing....could be related. Anyway if anyone has any info please help me out. I am waiting for the vet to get back, but she must be busy or on her day off. I was going to worm him, but have not yet to ensure that worms are not the cause of his weight loss. He is due to be wormed.