my husband is connected in a private company for 27 years.Since the first day of his employment , he was never paid double during holidays and he never avail incentive leave.Right now he was planning to resign. Can he claim holiday pays and incentive ?

As a general matter, there is no holiday pay or incentive leave in the law: employers are not required to provide this. If they do, it is voluntary. If they do and there is no written agreement or firm (no caveats, restrictions, limitations, etc.) written policy guarantying it, it is purely discretionary: the employee has no enforceable  right to it, but rather gets it only when the employer chooses to pay him--he can't make the employer provide these things.
If there is a written policy or agreement but it retains discretion for the employer, it is again up to them--there is no enforecable right.
If there is a written policy or agreement which contains limitations or restrictions, those limitations or restrictions are enforceable: he only gets them to the extent the policy allows them.
Only if there is an unrestricted or unlimited right contained in a firm written policy or written agreement, would your husband potentially have an enforceable right to this compensation, but even there, there is another factor: because whatever rights he has in this case are funamentally contractual (based on an explicit or implicit agreement with the employer), he can only enforce those rights for 3 years, which is the statute of limitations (SOL), or time to take legal enforcement action, for a contract. So best case, he can seek these benefits or pay for the last 3 years only.