I have worked out an answer for you below ...
1. While a seizure is always possible, in a Boxer of ten years of age this sort of sudden onset incoordination and collapse syndrome is more often associated with some sort of chronic heart problem, particularly if your dog was having strenuous exercise at the time.
2. Sadly with whatever is going on it would be very unlikely that you will be able to handle this yourself on a home remedy basis so all you can do is to keep him comfortable and reassured until you can arrange to have a vet check him over.
3. Even if this turns out to be heart related then there are a range of prescription heart drugs which can often greatly help.
Your first move though is to have a vet check him over so you are sure exactly what you are dealing with ...
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