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Sorry to hear about what is going on.
There are three main areas to consider. The first is infectious disease like lyme. This will hit quickly and cause tiredness, lameness, not wanting to eat, and a fever.
Secondly, you could be dealing with heart disease. Often they don't show any symptoms until it hits them hard. But, if the heart is not functioning well, there can be extreme weakness to the hindend, and shortness of breath.
Also, this could be extreme arthritis and muscle weakness, which he just can't compensate for anymore.
You need to see a vet to make certain this doesn't become more serious. All of these things can be treated, or managed. Please see your vet, and let me know what they find.