Severance Pay Negotiation

My husband has worked for his company since 1989. 6 years ago he left the company and was rehired and reinstated for all benefits. When he returned, the company gave him a letter stating that his adjusted rehire date is now in 1990. Now they are terminating him and using the 6 years ago rehire date as his length of service for calculation of severance pay which makes him eligible for 12 weeks. Their severance pay policy summary states that employees will be given two weeks severance pay per year of credited service up to a maximum of 26 weeks. He has 19 years of credited service with the company but they refuse to offer any more. Do we have a case? What is our recourse? How do you recommend we go about getting this resolved?