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Without further information it is difficult for me to provide the best advice, though I will certainly try. Please respond when you can and we can continue the chat.
Lethargy, a reduced appetite and lameness can have many causes including:
A vet visit would be wise so he can be examined and as we may need to e.g. run some blood tests or take an x-ray.
I suspect he will benefit from some medicine such as pain relief and anti inflammatories.
Try to tempt him with warm and tasty food such as chicken.
If his gums are pale or dry, he has a bloated abdomen or fast breathing, he is becoming more lethargic or you feel he is really not right, he should be seen by an emergency vet tonight.