WInd puffs is a term used to mean a stretched area where fluid from the tendon collects and is a term associated with the fetlock area of the horse. They usually come up in young horses in training and is a warning sign of excess strain. They just look like tiny bumps on either side of the fetlock joint, usually on the higher part of the joint. THey are soft swellings, and can cause little to no lameness at the time. They don't go away..it's (and I'm just making a comparison here, it's not exact) like a varicose vein, only in this case it's not a blood vessel that sags, it's the tendon sheath. Besides rest or cutting back on the work load in the immediate time frame and maybe some icing/hosing at the time, they are pretty much a permanant blemish, but usually don't cause any problems.
What I see on your horse is a "popped knee' . I'm just going by some photos, and I'm NOT a vet. But popped knees are more serious as that's going to continue to calicfy and stiffen that joint over time, seriously limiting his mobility. Arthritis is going to be a major issue as well. You see this with horses that are "calf kneed" or "back at the knee" more and esp if they have careers that pound the joints (racing, jumping, roping cattle). You may be able to manage it just fine and keep riding him, but it would be in his best intrest for you to NOT do a lot hard stops or any jumping. It's not just a "comsetic issue" as a wind puff would be. A lot of times the cartilage and ligaments are damaged or it's a bone chip and the leg is attempting to heal it by calcifying the whole joint. If it were his hock , we'd call it a "bone spavin" as the way the joint tries to heal itself is very similar. This is a serious issue, not cosmetic. It will interfere with the mechanics of his leg, like fusing your wrist together.
I trim a really old horse that has both knees that look like this(he used to be a roping horse and had "calf knees") and I literally have to sit cross legged on the ground and can only pick his feet inches off the ground to trim, because they don't bend any further and he will collapse if you try to make them. I have to prop his foot on a tiny block of wood and work one handed. He can swing his legs fully forward, he just can bend them back. He's very good about being patient as I work in that awkward angle. The owners ride him some..just walking now. He's old, retired and is more pasture mate than anything.
Turnout and walking is good exercise to help with the arthritis.Making sure his hooves are trimmed and the toes never get long will help. I hope you can find more information and have everything I mentioned checked out with a vet , of YOUR choosing. X-Rays are in order.