"He does not have to go along today."
Translation:Hij hoeft vandaag niet mee te gaan.
First of all "Hij mag niet" means "he's not allowed to" or "he may not".
Second of all in Dutch we have some auxiliary verbs that can be followed by a bare infinitive and can't be followed by a bare infinitive, for those words we use "te" before the infinitive (Kind of the same in English: you can say "I want to sing" not "I want sing". On the other hand, you can't "I can to sing" because 'can' is normally followed by a bare infinitive). "Hoeven" is a verb that needs to have "te" before it and "mogen" doesn't. So the grammatical correct sentence in Dutch would be "Hij mag niet mee gaan vandaag" but then again see my first point