Hello and welcome to Just Answer
I am Dr Z and I will try to to help you.
Will he let you look in his mouth at all?
How far is his tongue staying out (a little or a lot)
Is he drooling as well?
Is he allowed outside or is he indoors only?
Ok, that reduces the liklihood of trauma a little, but still a possibility.
How are his lips? Do they look normal especially at the curve where the upper meets the lower lip?
What I would be most worried about is trauma either in his mouth or under his tongue.
Cats, being the curious beings they are, often end up ingesting or chewing on things they shouldn't.
Something may have punctured or even wrapped around his tongue base on the underside.
This is actually a common problem. It is difficult to see on an awake cat.
The smell makes me even more worried. There may be dead or diseased tissue if there is infection.
Yes, that would be best.
They may need to sedate him, but they should get a good look then and hopefully can help him