Why is is hen instead of he? At the end?
That's the plural form used when the subject is plural. Here, 'वे लड़के' (those boys) is plural so you need to use it.
Also, it shouldn't be pronounced 'hen'. The dot, in this case, is telling you that the vowel ै needs to be nasalised.
and why do I need to write hen? I thought when there is a full verb at the end of the sentence, you do not need to write a form of hona? I left it out and was told that this was wrong...
You need a form of है in all sentences in the present tense. The exception is negative sentences with a full verb.
Thanks! I got that wrong. Really great help to have people like you around.