Landlord keeping security deposit - his lawyer claiming invalid demand letter

I mailed my Landlord a 30 day notice about him taking my security deposit. He violated several sections of the law so I would assume this would be an easy win in small claims court (though I've never filed a case before). He hired a lawyer which mailed me back saying my demand letter is invalid. Is there such a thing? My Demand Letter was tailed from Mass. Legal Help which is highlighted on Mass.gov.
It's been nearly two years so I'm worried the Statute of Limitations could run out. Can anyone help me out.