What happens if I uncheck a box for retirement plan that a W-2 has incorrectly checked?

My husband worked for company A until Dec. 2017 and had a retirement plan.  He went to work for company B in Jan. 2018 and has no retirement plan. Company A paid his last paycheck for work from 12-15 to 12-30-17 in Jan. 2018 and contributed $284 to his 401K.  The box for Retirement Plan is checked on his W-2 which keeps him from contributing the max to his IRA because it looks like he was in a retirement plan for part of 2018.  His income was earned in 2017 so he was never in a retirement plan in 2018.  Because of this he can only deduct $2250 of a $6500 contribution to his IRA.  If that wasn't checked, then he could deduct $6500.  Is there any way around this?