legally you are entitled to 50% of his 401K if

if you lives at PA, WA. Other states may have different law. Phone his company's HR. They should tell you the law. If you know the company manages his 401k, call there too.

I know in PA and WA, unless the spouse signs some paper to say that "I don't want share his 401k", the spouse automatically shares 50% of the 401k.

That is YOUR money. GO for it, girl!