I believe, but am not certain, that the English is not correct here. Just as the Spanish is using a subjunctive, so should the English.
Asi: "It is necessary that he travel more."
Creo, pero no soy cierto, que el ingles no es correcto, aqui. Tal como el espanol esta usando un subjunctivo, por lo que debe el Ingles.
See these sites (Ver a estos dos sitios): 1) http://usefulenglish.ru/grammar/present-subjunctive 2) http://faculty.washington.edu/jwhelan/Documents/Drill%20Sheets/Subjunctive.html
(No un hablante nativo de Espanol, pero intenterando a aprender.) Hice informe al Duolingo.
"Necessary" is one of those "trigger words" that trigger a subjunctive, both in English and Spanish.
The correct English here is "necessary that he travel more." In English, the subjunctive is given by the "base form" of the verb, the infinitive without the "to." Thus, "travel" is the base form, and is the correct subjunctive.
Correct English is: "It is necessary that he travel more."
The Spanish is correctly the subjunctive.
I reported the mistake.