He may have been sick while you had the game closed. Occasionally they will recover from an illness on their own, without the meds from the store. His health and energy levels would have dropped due to the illness and just haven't had time to rise yet, but there would be no message of illness.
As Lyssa posted, you just need to make sure there is plenty of food and he is eating meals and his health should rise over time. The store items (fruits, hand wash, vitamins) are a quick health boost if/when they show up.
The only thing that concerns me is that his health is continuing to drop. That is strange, if all the above has been done and the doc consult is clear. Try closing the game and restarting it to be sure there isn't an "invisible" illness that may show up on relaunch.
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