If a teen earned less than $900, does the teen need to file taxes?

I did not need to file taxes for this situation (years ago) I do not think. Here is what I found:

You must file taxes if you earn the following amounts of income:

Self-employed, any age: $400
Children and Teens classified as a dependent: $5,700

(answers.com)

He must file if his earned income was more than 5700 (wages and taxable scholarships and grants), his unearned income was more than 950 (interest/dividends), or his self employment income was 400+ (1099 MISC).

It might be in his interest to file if he is due a refund vs. including it in your income and claiming him. Try it both ways.