Learn French: Il dit non.

It comes from the verb dire, which means to say or to tell. While it belongs to the 3rd group of verbs because it ends in -re, it is highly irregular. It does not follow the standard endings you learn for regular -re verbs like attendre or répondre. For il (he), the present tense form is dit.

Is the 't' at the end of dit pronounced?

No, the t at the end of dit is silent. You just pronounce it like the English letter D. In French, final consonants are usually silent unless they link to a vowel in the very next word.

Does this mean 'He says no' or 'He is saying no'?

It can mean both! French does not have a separate tense for actions happening right now. The simple present tense in French covers both 'He says' and 'He is saying', so you will rely on context to know which one is meant.

Could I use the verb parler instead of dire here?

No, because parler means to speak or to talk (like having a conversation). Dire means to say or to tell. When you are expressing the specific words someone says, like non, you must use dire.