Try this: estimate.fieldsToNull.Include_in_Oppty_Rollup__c=(val) ? true : false;

Probably it is throwing that error because you are using 'True' and 'False'. Those values are of type String. Remove the '' to use Boolean instead of String.

Try this: estimate.fieldsToNull.Include_in_Oppty_Rollup__c=(val) ? true : false;