Ok, Mike, there are a few things that I would consider in a Boxer dog this age.
1. He may have stretched or ruptured his cranial cruciate ligament.
This is something humans get as well, but in humans it is called Anterior Cruciate Ligament.
THIS IS WHAT I FEEL IS MOST LIKELY FROM YOUR DESCRIPTION!
The cranial cruciate ligament is one of the structures that stabilizes the knee joint. It is prone to problems, especially in large breed dogs. If the dog is in motion, and the lower leg is held still (by going down a hole, getting caught on a string, etc) while the upper leg keeps moving forward, there will be damage to this cranial cruciate ligament.
It is by far the most common injury that I see causing the symptoms you are describing!
The symptoms seen are usually a sudden onset of hind-end lameness, with toe touching seen at standing and walking, but the dog often carries the leg when running.
Here is more information about this problem:
And you can see more here in this video:
2. Your boy may have a fracture (broken bone) of one of the bones in the foot.
I have seen this many times with a dog getting their toes jammed under the door of a crate or between the planks of a deck or similar, and then twisting somehow and causing a fracture, usually in the foot area. Symptoms are a sudden onset of non weight bearing lameness, just as you are describing. Usually, you can see swelling on the foot, and the dog reacts when touched on the foot.
X-rays would help to diagnose this.
Here is more: http://www.petplace.com/dogs/fracture-of-the-metatarsus-and-metacarpus-in-dogs/page1.aspx
3. It is also possible that your dog has dislocated his hip.
This is a common dislocation and again could happen if he trapped his toes under the door and then pulled. It is very unlikely but certainly possible. With this, again you would have the symptoms of non-weight bearing lameness. Here is more:
So, in summary, there are a number of different orthopedic conditions that can all give these same symptoms you are describing in your dog. In order to diagnose this, a veterinarian would have to perform a thorough orthopedic examination, and a neurological exam.
The vet might well suggest an x-ray.
For this part, it might be necessary to give your dog a pain killer to allow proper positioning for the x-ray without hurting him. I do think your dog needs to see his vet to have this looked at. Something is going on, and the sooner it is diagnosed and treated the better.
If this persists, I definitely would recommend a visit to see your veterinarian.
If your boy is eating and drinking normally, then you could try conservative treatment: STRICT rest for 2 weeks! That means no playing with your other Boxer, no walks, no ball playing, no jumping - just REST for 2 weeks. I know that this is hard to do with a 2.5 yr old bundle of energy!
Be particularly careful with him if he has to go UP stairs as this is when all his weight is on the back legs and he is more likely to do additional damage.
To help your dog to get up the stairs, I would recommend using a "sling" that you can make yourself. Take a long bath towel, and fold it 3 or 4 times lengthwise, so that you have a long and thin shape. Put this under his belly, as far back as possible, so it is under the hips. Now, bring the ends up over the back so that you can hold the ends above him. With this, you can lift almost all his weight by using this sling, and thus really help him up the stairs. He can walk himself, with you right beside him taking his weight on the sling to help. This is a really good way to help your dog up stairs or up into the car - or any time he has to go UP!
You may be able to give some aspirin (as long as he has no history of kidney problems, stomach problems and is not on other medications). Please use Buffered Aspirin if you have it, and give it with a piece of bread or something low-fat to eat, not on an empty stomach.
Here are links that tell you about it, with precautions and dose:
I would suggest that at most you use this at most for 2 or 3 days. Also, do mention to your vet that you gave it.
If your boy is not improving after 2 weeks of STRICT rest, or if he is getting worse, then you should see your vet to have him evaluated.
I hope that this helps you to help your dog!
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The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would strongly advise contacting your regular veterinarian.