Invalid Separation Agreement?

My husband and I made out a separation agreement.  He refused to get a lawyer stating he didn't want to waste the money.  My lawyer drafted the document and it was signed and filed by both of us.  In the agreement, my husband gave me the house (signed the quit claim deed and I refinanced), he pays nothing but health and car insurance for the two kids (one in high school other in college-tuition in mortgage I'm paying).  No child support or alimony.  He then lost his job and is stating that the agreement is "Invalid" and is saying he wants "something" from the house since this new job pays less and his standard of living is "changed".  We have not yet been to court.  Can he do this? – Kat