Learn French: Il met de l'ail dans tous ses plats. - He puts garlic in all his dishes.

Because garlic is treated as an uncountable quantity here. You are saying he puts some garlic in his dishes. In French, you must use a partitive article like du, de la, or de l' for uncountable things. Since ail starts with a vowel, it becomes de l'.

Do you pronounce the s at the end of tous in this sentence?

No, the s is silent here. When tous is used to describe a noun, as in tous ses plats, the s is not pronounced. You only pronounce the s when tous acts as a pronoun standing completely on its own, like if you were to say he ate them all (il les a tous mangés).

Why does met not end with an s?

The word met is the he/she/it form of the irregular verb mettre, which means to put. While the I and you forms end in an s, like je mets and tu mets, the he form drops it to become il met.

Could I use assiettes instead of plats to say dishes here?

No, because assiettes specifically refers to the physical plates you eat off of. The word plats is used for the actual prepared meals or recipes, which makes sense here since he is adding garlic to the food itself.