Bonjour Kiattiyos !
"Il sent mal" could only be used in a very specific context to mean "He smells wrong", i.e. that person has a problem with their sense of smell.
To say "He smells bad" (i.e. he stinks), you would use "Il sent mauvais", and to say "He feels bad", it would be "Il se sent mal".
Have a look at the related lesson: https://kwiziq.learnfrenchwithalexa.com/revision/grammar/how-to-use-sentir-bon-versus-se-sentir-bien
I hope that's helpful!
À bientôt !
