Is this the usual way?

Sorry that the question title is so vague, but anyway:

I'm a hafu who is currently penpals with a male Japanese student about 1 year older than I am.
While I use somewhat formal Japanese, 私、です、−ます,etc., he uses such Japanese as すんでるよ、だ、ーないな。。。Etc.

Do teenage boys in Japan usually talk this way to a younger girl?

Yes, they often talk so to their friends from the first, especially on the internet.
That's just friendly expression, not means to look down.
I think to use formal Japanese at first is good, but some think it's uncomfortable.

BTW, 先輩(senpai; senior) talk friendly and 後輩(kouhai; younger) talk politely is common in Japan.
Although I think your case is just only friendly expression, so you can talk friendly to him.